Customizable Embedded Sensors

ABSTRACT

A method of constructing a sensor includes depositing a first material in a predetermined arrangement to form a structure. The depositing results in at least one void occurring within the structure. The method further includes depositing a second material within the voids. The second material may have electrical properties that vary according to deformation of the second material. The method also includes providing electrical access to the second material to enable observation of the one or more electrical properties. A sensor includes a structure that has one or more voids distributed within the structure. The sensor also includes a material deposited within the one or more voids. The material may be characterized by one or more electrical properties such as piezoresistivity. The sensor includes a first contact electrically coupled to a first location on the material, and a second contact electrically coupled to a second location on the material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/627,745, filed Sep. 26, 2012, which claims the benefit of thefollowing patent applications, the contents of which are herebyincorporated by reference in their entirety: U.S. Provisional PatentApplication Ser. No. 61/650,531, filed May 23, 2012 and U.S. ProvisionalPatent Application Ser. No. 61/539,198, filed Sep. 26, 2011.

BACKGROUND

Creating custom sensorized structures for the human body can be time andcost intensive, and in many cases each particular user requiresredesigning it from the start. Streamlining the capability to embedsensing elements into structures can provide greater quantitativefeedback to users, medical practitioners, researchers for ergonomiccomfort, patient exercise progress during a physical therapy regime,monitoring tools for patient evaluation, and assistive tools worn dailyto improve quality of life.

SUMMARY OF THE INVENTION

In one aspect, the invention is a method of constructing a sensor. Themethod includes depositing a first material in a predeterminedarrangement to form a structure. The depositing results in at least onevoid occurring within the structure. The method also includes depositinga second material within the voids. The second material has one or moreelectrical properties that vary according to deformation of the secondmaterial. The method further includes providing electrical access to thesecond material to enable observation of the one or more electricalproperties.

In one embodiment, the depositing a first material further includesusing an additive manufacturing technique. In another embodiment, thepredetermined arrangement includes a sensor design. The predeterminedarrangement may include a plurality of consecutive layers. Each of theconsecutive layers may be a cross-sectional profile of the sensordesign. The voids may be defined within the profile.

In another embodiment, the second material is a conductive elastomer.The one or more electrical properties may include piezoresistiveproperties. The piezoresitive properties may include a change in theelectrical resistance characteristic with respect to the amount ofdeformation experienced by the material. The piezoresistive propertiesmay be associated with a piezopelectric effect. The second material maybe a room temperature vulcanizing silicon suspension of electricallyconductive particles. The electrically conductive particles may includenickel-coated graphite particles.

In another embodiment, depositing the second material further includesinjecting the second material through a port in the structure. The portmay provide access to the at least one void. In one embodiment, theinjecting is accomplished with a syringe. The syringe may connect to theport through a coupler that is securely fastened to the syringe and theport. The coupler may be s a Leur lock that has threads for coupling tothe syringe.

In one embodiment, providing electrical access to the second materialfurther includes attaching a first electrode to a first location on thesecond material and attaching a second electrode to a second location onthe second material. The first location may be a first end of the secondmaterial. The second location may be a second end of the secondmaterial.

Another embodiment further includes embedding one or more electricalcomponents in the structure. The one or more electrical components maybe electrically coupled to the second material. The one or moreelectrical components may be an amplifier, a filter, a comparator, anelectrode, a voltage regulator, a current regulator, a sampler or abuffer, or any combination of these components. In general, an componentknown in the art may be included in the structure.

In another aspect, the invention includes a sensor. The sensor includesa structure, and the structure may include one or more voids distributedwithin the structure. The sensor also includes a material depositedwithin the one or more voids. The material is characterized by one ormore electrical properties. The sensor further includes a first contactelectrically coupled to a first location on the material, and a secondcontact electrically coupled to a second location on the material.

In one embodiment, the structure includes a plurality of consecutivelayers, each of which is a cross-sectional profile of the structure. Theplurality of consecutive layers was produced using an additivemanufacturing technique.

The structure may be based on a sensor design, i.e., the structure isconstructed according a design plan created by a human designer, acomputer-based algorithm or other automated system, or a combinationthereof. In one embodiment, the sensor design describes a torque sensor.In another embodiment, the sensor design describes a force sensor. Inyet another embodiment, the sensor design describes an impact sensor. Inanother embodiment, the sensor design describes a bend sensor. In afurther embodiment, the sensor design describes a vibration sensor.

In one embodiment, the first location on the material is a first end ofthe material and the second location on the material is a second end ofthe material. In another embodiment, the one or more electricalproperties includes piezoresistive properties.

In an embodiment, the material is deposited within the one or more voidsby injecting the material through an opening in the structure. Anadapter connecting the opening to an injector may be used. The adaptermay include threads that are used to couple to the injector. The adaptermay be removably coupled to the opening in the structure, so that theadapter may be detached from the opening after the material is depositedin the void. In one embodiment, the material may include graphiteparticles in a silicone RTV suspension.

In another aspect, the invention is an orthotic device. The orthoticdevice includes a structure for providing support to a portion of humananatomy. The structure may include one or more voids distributed withinthe structure. The orthotic device may include a material depositedwithin the one or more voids. The material may be characterized by apiezoresistive property. The device further includes a first contactelectrically coupled to a first location on the material, and a secondcontact electrically coupled to a second location on the material.

In another aspect, the invention includes an ankle-foot orthosis, whicha structure for providing support for one or more of a foot, ankle andlower leg. The structure may include one or more voids distributedwithin the structure. The orthosis also includes a material depositedwithin the one or more voids. The material may be characterized by apiezoresistive property. The orthosis may include a first contactelectrically coupled to a first location on the material, and a secondcontact electrically coupled to a second location on the material.

In another aspect, the invention includes an upper extremity measuringdevice, which includes a structure having a first surface and a secondsurface. The structure may include at least one void distributed withinthe structure beneath the first surface and at least one voiddistributed in the structure beneath the second surface. The devicefurther includes a material deposited within the voids. The material maybe characterized by a piezoresistive property. For each of the voidswithin the structure, a first contact may be electrically coupled to afirst location on the material, and a second contact may be electricallycoupled to a second location on the material.

In another aspect, the invention includes a device for sensing contactwith an object. The device includes a structure having an exteriorsurface, the structure including at a first void and a second voidextending into the exterior surface. The structure includes a pluralityof consecutive layers, each of which is a cross-sectional profile of thestructure. The device further includes a material deposited into thevoids, wherein the material is characterized by a piezoresistiveproperty and wherein the material deposited into the first void is notin contact with the material deposited into the second void. The devicealso includes an electrical circuit electrically coupled to the materialdeposited into the first void and to the material deposited into thesecond void. The exterior surface contacting the object causes theelectrical circuit to form a closed electrical circuit. In oneembodiment, a conductive object causes the electrical circuit to form aclosed electrical circuit when the conductive object is electricallycoupled to the material in the first void and to the material in thesecond void. In another embodiment, the object causes the electricalcircuit to form a closed electrical circuit when the object manipulatesa cantilevered portion of the material in the first void to beelectrically coupled to the material in the second void. In oneembodiment, the plurality of consecutive layers was produced using anadditive manufacturing technique.

In another aspect, the invention includes a device for supporting atleast a portion of an electrical circuit. The device includes astructure including one or more voids distributed within the structure.The structure includes a plurality of consecutive layers, each of whichis a cross-sectional profile of the structure. The device furtherincludes a material deposited into the at least one void. The materialis characterized by a piezoresistive property. The material iselectrically coupled to the electrical circuit, such that the materialforms at least a portion of a conductor in the electrical circuit. Inone embodiment, the plurality of consecutive layers was produced usingan additive manufacturing technique.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other objects of this invention, the various featuresthereof, as well as the invention itself, may be more fully understoodfrom the following description, when read together with the accompanyingdrawings in which:

FIG. 1 shows regions of the human body that could benefit from deviceswhich combine 3-dimensional (3D) scanning and embedded sensors.

FIG. 2 shows the process of integrating design of the device withsensors and electrical wiring.

FIG. 3 illustrates the Konica Minolta Vivid 910 Laser Scanner.

FIG. 4 shows the progression that occurs when creating a digital versionof the surfaces and colors of a physical objet using non-contactstereoscopic photogrammetry.

FIG. 5 illustrates generic AM processing.

FIG. 6 provides an SLA illustration.

FIG. 7 illustrates an FDM setup.

FIG. 8 illustrates a multi-polymer jetting process.

FIG. 9 illustrates an exemplary SLS system.

FIG. 10 illustrates an exemplary strain gauge.

FIGS. 11A and 11B present different research and commercially availablesensor techniques that use the piezoresistive effect.

FIGS. 12A and 12B present different research and commercially availablesensor techniques that use IR/optical for human biomechanics sensing.

FIGS. 13A and 13B present different research and commercially availablesensor techniques that use conductive materials for human biomechanicssensing.

FIG. 14A illustrates fitting of components between stages.

FIG. 14B illustrates SDM stages of Robotic Insect Body with embeddedcomponents.

FIG. 15 presents different research and commercially available sensortechniques that use small scale mechatronics for human biomechanicssensing.

FIG. 16 illustrates devices that use a spring flexure element built fromAM materials.

FIG. 17A illustrates the general working principal for piezoresistiveelastomer suspensions.

FIG. 17B illustrates electrical response during tensile testing.

FIG. 17C illustrates Gage Factor plot for conductive silicon.

FIG. 18A illustrates longitudinal strain for polymer bridge.

FIG. 18B shows polymer bridge key dimensions and composite equivalency.

FIG. 19A illustrates a force sensor according to the describedembodiment with a Leur lock.

FIG. 19B illustrates MPJ sample sensor and load cell responses to 6 Hzsinusoid.

FIG. 20 is an example of such a commercial (Futek) torque sensor.

FIG. 21 shows a polymer acting like an instrumented shear pin betweentwo zones.

FIG. 22 shows a handle design.

FIG. 23 illustrates a alternative version of the hydraulic handledepicted in FIG. 22.

FIG. 24 illustrates the fabrication and injection stages of the handledepicted in FIG. 23.

FIG. 25 shows channels embedded inside a thimble switch.

FIG. 26 shows a two part custom-designed wrist-mounted electronic devicewith embedded channels.

FIG. 27 illustrates a process for creating a custom RP AFO.

FIG. 28A illustrates the flow diagram of digital processes for pointcloud refinement.

FIG. 28B shows the overview of the sensor and AFO functions interactingwith the wearer.

FIG. 28C illustrates a process diagram for creation, instrumentation,application and logging of a custom sensorized AFO.

FIG. 29 illustrates the posterior view of AFO CAD.

FIG. 30 provides a comparison of three AFOs.

FIG. 31 shows the feature detail for the channel injection sites on theAFO instrumented according to the described embodiments.

FIG. 32 illustrates a robot wing with an embedded strain sensing and arobotic leg with embedded sensors to detect impact from ground reactiveforces.

DETAILED DESCRIPTION

Suitable exterior regions where custom structures which need to support,sense, and interact with the body exterior can be determined based onthe following four criteria:

-   -   Regions that are most utilized for the largest variety of        activities of daily living.    -   Controlled range of motion is used daily for best quality of        life.    -   Regions that have a large anthropometric variability—not just in        terms of key measurements but of curves and surfaces.    -   Boney prominences where the tissue can be uncomfortable if        pinched, or where loss of circulation can occur if        over-compressed.

FIG. 1 illustrates regions of the human body that could benefit fromdevices which combine 3-dimensional (3D) scanning and embedded sensorsin, for example, an additive manufacturing structure. As shown, theankle-foot complex 102, the wrist-hand complex 104, and the neck-headcomplex 106 exhibit these characteristics. The exemplary embodimentsdescribed herein generally focus on the first two regions: the distaleffectors, using requirements and results of sensorized tools for theupper extremity as example sensor modalities. The head and neck complexis included within the scope of the appended claims and of thisdisclosure, even though the head-neck complex is not explicitly treatedin the exemplary embodiments. Further, other regions of the body mayalso be considered to be included within the scope of the appendedclaims and of this disclosure, even though not explicitly treated in theexemplary embodiments.

A piezoresistive sensing phenomenon is used as the transducing elementof the described embodiments. Such a transducing element has differentmodalities depending on what the physical phenomenon is to be sensed. Afamily of sensors is possible using this underlying transducing element,and examples are shown that may be used to measure force, pressure,torque, vibration, impact, and contact, among others, and combinationsthereof. Additionally, the sensor design and fabrication options allowthe user to tailor the sensing range depending on the application on thebody and the anticipated loading magnitudes.

The described embodiments may customize the characteristics of eachforce sensor, for example, to the individual user and surroundinggeometry in the device via two implementation modes:

-   -   Intra-Device Phenomenon: Self-diagnostic measurements for health        of the device containing the sensor, such as: impact, mechanical        fatigue, device damage, device flexion. One example includes an        ankle foot orthosis with embedded sensors to check for wear &        tear, impact sensors detecting a force threshold.    -   Device-Environment Phenomenon: Sensing interactions between the        surrounding environment/person, and the device body. One example        of this implementation mode includes computer-interface button        device, detection of heel strike and contact in footwear,        wearable medical monitoring tools, torsion and force sensors.

The described embodiments combine sensor design and device design toaddress needs not previously researched. Customizable force sensors havebeen discussed in previous work, but limited mostly to the nano andmicro-scale. In order to effectively interact with the forces of a humanbody, meso-scale instrumentation is required. As used herein,“meso-scale” means physical dimensions able to be estimated by the nakedhuman eye. Similarly to off-the shelf sensors, the unique geometry ofthe disclosed embodiments, and their scalable design requires veryspecific fabrication capabilities which can also easily produce a widerange of sensors—both stand-alone sensors and sensors that are embeddedin the body of a device itself. This necessitates a fabricationmethodology which can build highly accurate features at a small scale;hollow features & voids; thin surfaces; and is expandable for a masscustomization platform. It is feasible to fabricate these structuresusing several methods including insertion casting, lost wax casting,5-axis precision CNC milling, or plastic dipping.

For speed and process flexibility, the exemplary embodiments describedherein utilize the technique of Additive Manufacturing (AM), alsocommonly referred to as ‘Rapid Prototyping’ or ‘layered manufacturing’.AM differs from conventional subtractive fabrication methods likemilling and turning because it creates three-dimensional contours andfeatures by adding and bonding successive thin cross-sectional layers ofmaterial rather than removing material from an initial structure ordeforming portions of the initial structure.

The unique advantages of AM have been adopted by medical practitionersin surgical theater as tool guides and surgical implants because oftheir fabrication flexibility and production speed. For the same reasonsAM is also being explored as a way to build functional plasticcomponents for the exterior of the body. Developments in non-invasive 3Dscanning have made it possible to acquire digital models of freeformsurfaces like the superficial contours of the human body to serve as thedesign references. The combination of these two technologies can providepatient-specific data input corresponding to anatomical features; aswell as a means of producing a readily-instrumented patient-specificform output with electronic components already embedded. In the medicalcontext AM sensorized devices are well suited to assist, measure &evaluate, and rehabilitate patients.

FIG. 2 shows the process of integrating design of the device withsensors and electrical wiring. The process begins with Concept andDesign 202. By using a flexible fabrication like AM, the design inputparameters range from anatomical landmarks and parametric equations toqualitative usability selections by the end user or medicalpractitioner. Following Concept and Design is computer-aided design(CAD) processing 204, during which a CAD model is generated for thestructural shape with specialized cavities, voids, channels and othertypes of empty features (referred to herein generally as ‘voids’) builtin to accept the conductive elements. During the Build stage 206, AMtechniques are used to produce a physical manifestation of the model.Once the fabrication has been completed in the Build stage 206, theseempty features (e.g., voids and cavities) are filled with a conductivematerial (for example a piezoresistive elastomeric suspension) duringthe Inject 208 stage. Depending on the specific geometry for each of thevoids, once filled they will respond to mechanical stimuli differently.The complete device is then able to function 210 as it responds tointeractions with the user and itself. The responses range fromconducting electricity to increasing resistance from applied force,torsion, or moments, as originally planned by the selected internalgeometry of the cavities.

There are over 700,000 incidences of stroke each year in the US. Duringthe acute care phase following injury to the brain like stroke or impacttrauma, physical therapy exercises for fine motor control, dexterity,and gait initiates functional recovery via neuro plasticity.

Reduced sensitivity and range of motion of the affected side of the bodypresents challenges in monitoring a patient to ensure that they areresponding effectively to a rehabilitation regime. Assessment scales forpatient recovery are based off capabilities similar to activities ofdaily living like turning a doorknob or opening a jar and usuallyinclude patient-based feedback on a discrete scale. A family of easilysensorized tools could compliment this process which can track patientprogress in a quantifiable way. Tools which can connect to a computer orelectronic storage device can record patient activities and exercisesduring and outside of the scheduled physical therapy sessions and havebeen shown to improve motor function.

For conditions remaining past acute care, long term use of assistivedevices like orthoses or braces enable users to have a wider range ofactivities of daily living (ADL) for a higher quality of life. Suchdevices have user-specific features since their efficacy of thesedevices is affected by how well they suit the specific conditions thepatient's anthropometry and isokinetic capabilities, which can varysignificantly according to the specific motor control capabilities. Byembedding sensors in these devices, clinicians will be able to monitorthe wearer's daily activities in and outside of the hospital setting.

3D scanning devices digitize freeform surfaces by capturing a cloud ofdiscrete coordinates. The points are connected to re-create surfaces ofthe scan target digitally. Depending on the resolution of the scan, theprocess can be used for industrial manufacturing quality control ofdimensional accuracy and surface roughness, digitizing legacy componentswhich have no dimensional documentation, or medical modeling where theyare used as the intermediate step to fabricate anatomy via additivemanufacturing. Such models have had successful implementation inpreoperative planning, custom hip and knee implants, facial prostheticspost-surgery and teaching tools for patients and medical staff. Forsubcutaneous imaging, Computed Tomography (CT) and Magnetic ResonanceImaging (MRI) have been used to generate the points and surfaces of thetarget, but this discussion will focus on technology which capturessuperficial features only. These devices may be divided into twocategories of contact and non-contact.

Contact devices physically touch the surface and register the locationby deflection at the end effecter via electronic switch. Contactingtouch-probes vary in their resolution from sub-millimeter scale to mesostructures and are often very accurate over a wide measurement volumebecause they are often in the form of an end effector and articulatedarm that provides a mechanical ground relative to previous measurements.

Non-contact scanners (i.e., 3D laser-based scanners) are able to capturesurface geometry from a distance. Depending on the technology can beonly a few centimeters from the surface, to several miles in the case oflandscape mapping. For softer and more delicate structures or for verylarge objects (an aircraft, for example) they sweep across the field ofview with light-based measurements. In some cases handheld devices useboth methods to capture a larger field of view with mechanicallygrounded coordinates attached to the point cloud.

3D laser-based scanners are non-contact scanners that emit a beamoriented normal to the surface to be scanned. The light reflected backfrom the surface is captured as a 2D projection by an imaging device(e.g., a Charged-Couple Device (CCD) camera) and a point cloud iscreated using triangulation between the two cameras and the laseremitter. Laser scanners are designed for contour capture and cannotrecord color or texture information without an additional image to wraparound the digital surface. FIG. 3 illustrates the Konica Minolta Vivid910 Laser Scanner, an example of a laser scanner suitable for use withthe described embodiments.

For medical applications, most commercial laser scanners used forscanning are rated as eye safe, but reflections on curved surfaces andother inadvertent events can result in a potentially harmful focusedbeam. Knowing the receiver locations and orientations, lines aremathematically triangulated to produce 3D coordinates of each unobscuredpoint in both pictures necessary to reproduce an adequate point cloudfor shape and size reproduction. 3D photogrammetric scanners use imagescaptured from different points of view to reconstruct a surface. Imagesare taken from at least two different known locations in order totriangulate and measure “lines of sight” for each targeted surface. Theimages in FIG. 4 show the progression that occurs when creating adigital version of the surfaces and colors of a physical objet usingnon-contact stereoscopic photogrammetry. In this case of measuringsurfaces distances in three dimensions, there must be two points ofreference (i.e. two camera receivers) to determine the depth of each XYcoordinate. This is analogous to humans' depth perception by having twoeyes. This device projects a color map onto the scan target and capturesthree JPEG images—one from each camera while the projected color map ison, and a single image when it is off.

To turn the point cloud into a useable reference for Computer-AidedDesign (CAD), two intermediate steps take place. The first is to cleanthe point cloud by removing anomalies (spikes), filling holes, anddecimating the cloud to reduce the file size. The second step is to fitparametric shapes on the scan surface. For applications with mechanicalparts with parametric geometry it is possible to fit shapes likecylinders and cubes, but for the smooth freeform contours of organicshapes Non-Uniform Rational B-Splines (NURBS) are most appropriate.

The capabilities of designing devices and parts to fit anatomy aresignificantly affected by the quality of the scan data from the initialcapture. Scan quality of bare skin is greatly impacted by physicalcharacteristics of the subject, environmental conditions, and thecapabilities of the hardware used. Target and lighting conditions canreadily be controlled with appropriate calibration, consistent evenlighting and a stable scanner mount; but the greatest variation remainswith the subject. The average skin tone and hue vary from one person tothe next, as well as the variation within a patch of skin. These areconditions common in medical applications which pose challenges for 3Dscanners to accurately digitize superficial geometry of live humansubjects. Skin allows wavelengths of light and radiation to pass throughfor perform vital functions (e.g., the production of vitamin D). Much ofthe light emitted from a 3D scanner will experience sub-surface scatterwhen passing through the epidermal boundary and refract or be absorbedunder the skin surface which limits the number of data points registeredand generates errors spikes. For high-accuracy laser scanners, the bloodvessel and skin deformation from a single heartbeat may appear as twodifferent surfaces. Voluntary motions like the subject remaining still,as well as involuntary motions like heartbeat, twitches, or tremblingpose challenges for high-quality scans. For thinner anatomy like the earlobe these effects from the circulatory system are even more prevalentand can induce small, but uncontrollable scan deviations. Opticalscanners also have difficulty capturing sharp edges like individual hairfollicles which scatter light in random directions away from thereceiver. Large surface patches of missing data can result fromexcessive specular reflection, Even slight motion can cause anomalies asspikes in the mesh, and hair follicles, and certain tones can be morechallenging to record.

Scan quality from projected-light 3D scanners can be sensitive to colortone and specular reflection of a scan surface. Samples had outerdiameter 3 cm and leg length 2.5 cm. Scans were taken against a blackmatte background with leading edges 70 cm away from the central lens.The matte samples were evenly coated with Krylon Dulling Spray 1310(Krylon Products Group, Cleveland, Ohio, USA). Glossy samples wereevenly coated with Krylon UV-Resistant Clear Acrylic Gloss Coating. Thematte samples have higher diffuse reflections, and the glossy sampleshave higher specular reflection based on the refractive index of eachcoating.

Under all conditions, white surfaces have the best scan quality for evensurfaces. The black sections are unable to reflect sufficient light andhave the largest irregularities and spikes. The high-gloss edges aredifficult to register because they scatter light randomly and appear asholes. These factors must all be taken into account when designingdevices to fit on the human wearers to maximize their comfort andfunctionality.

An object with complex freeform 3D contours can be very challenging andvery costly to prototype & manufacture with traditional fabricationmethods. Additive Manufacturing (AM, but also known as “LayeredFabrication”, “Rapid Prototyping”, “3D Printing”, “AdditiveFabrication”, or “Layered Manufacturing”) is a fabrication methodologywhich opens possibilities to readily fabricate these previouslyimpossible features in a fast, accurate, and cost-effective way.Subtractive machining practices like milling and turning remove wastematerial until only the part features remain. AM fabricates athree-dimensional object from the base up by adding thin consecutivecross-sectional profiles of the object which bind together for acomplete 3D shape. This is fixtureless fabrication since no new toolingis required and although there are many different fabrication materials,machines, and procedures worldwide; the natures of these technologiesremain similar.

The unique capabilities of AM have benefitted the engineering designprocess in reduced development time & cost, greater variety in a familydesigns, and prototypes more accurate to functional testing of the finaldevice. The first RP machines began with 3D Systems in 1986, but thetechnology and industry have already made significant strides indevelopment in a relatively young life. The normally long time periodsbetween design iterations for form and fit evaluation can besignificantly reduced with AM, so depending on part size it may takeonly a few hours to go from digital design to physical part. Thesefactors make the technology excellent for custom parts produced to orderin small quantities. Virtually all layered processes can depositmaterial in the horizontal plane much more rapidly than they can buildup thickness. Consequently parts are typically built lying down so thattheir shortest overall dimension is oriented along the z-axis tooptimize for build time. Parts are also frequently nested within thebuild chamber to maximize parts per build cycle.

Summary of Advantages Using AM in the Medical Field

-   -   Highly accurate at small scale    -   Able to build hollow features and voids    -   Thin complex surfaces    -   Convenient mass customization    -   Consolidation of components    -   Decentralized distribution network    -   Complex geometry from biological and mathematical inputs    -   Functional materials    -   Small batch fabrication

Designing parts and shapes with curved internal features and undercutsoffers new advantages which are currently being researched as moreefficient hydraulic cooling channels and for safer routing ofelectronics. This eases part consolidation since build complexity isless impactful compared to subtractive fabrication methods. Reducing theparts in the bill of materials and integrating electrical components isone benefit of embedded semiconductors using techniques of the describedembodiments.

Part of concurrent engineering practices is considering the method andconstraints of fabrication at each phase of the design process.Traditional subtractive fabrication methods have a large emphasis on‘design for manufacturing’ and anticipating these constraints early on.Although AM processes do also have materials and geometry limitations,their capabilities encourage a reverse philosophy of ‘manufacture fordesign’. The generic AM process laid out in FIG. 5 can be a continuationof the 3D scanning process (starting at the CAD phase with parametricsurfaces) or can operate independently as any other fabrication stage.

The tool paths (or in some cases laser paths) are generated from theBuild preparation according to the surface geometry of the part and itssupport structures. The key digital step in this process is generatingthis surface geometry using a Standard Tessellation Language (STL) file.An STL file recreates the surface geometries of the CAD part intriangles of varying size and shape and does not contain any otherdesign data, just measurement units and geometry. Similarly todecimating and parameterizing the 3D scan surface, the resolution of thetriangles affects the file size, complexity, and physical resolution ofthe part to be built. For complex internal voids and features it is notonly important to have a fabrication method with high enough resolutionto build them, but that the STL file and build preparation are set topreserve these features and not interpolate them away. The processesdescribed below represent the current major groupings of commercialtypes of AM, with each one able to build internal features in flexibleplastic materials.

Stereolithography

Stereolithography (SLA) is a comparatively older technology than some ofthe other additive manufacturing processes, but it remains one of themost widely used methods. It was one of the first processes that couldproduce a part strong enough to be used as an end product rather thanjust a design mockup or a prototype.

The stereolithography process uses a laser beam in the ultravioletwavelength (on the order of 325 nm) to sequentially cure (polymerize)cross sectional slices in a liquid photopolymer resin vat to create the3D contours of the build object. See FIG. 6 for a SLA illustration. Thearea of photopolymer that is hit by the laser beam partially cures intoa continuous thin sheet which is parallel with the X-Y plane. Theplatform upon which this sheet sits is then lowered by one layer'sthickness (3D Systems Viper resolution is on the order of 0.05 mm in theZ-axis) and the laser traces a new cross section on top of the first.Most lasers are static in the machine, with the beam continuouslyredirected by mirrors for profiling the path. The build sequence for alaser is typically first the borders to dam the liquid volume fromflowing out, followed by a rectangular hatch to solidify the layer. Thelaser is powerful enough to penetrate through the top few adjacentsheets and binds them together to create the final three-dimensionalobject. Acrylate-based photopolymers are the most widely used resinsystems developed for stereolithography. As part of the buildcalibration, each part is scaled to account for a shrinkage factor,usually on the order of 0.8% or less.

For any overhanging features in the part a support lattice framework isbuilt with each layer to stabilize the part geometry and isolate thepart surface from the build platform. Inclines of greater than 30typically do not require a support structure.

After the build process has been completed, some post processing isrequired. The support lattice needs to be manually removed and thecontact surfaces manually cleaned. Isopropanol is a common chemical toassist cleaning. After cleaning, the part must be transferred to aultraviolet (UV) oven to finish curing the resin.

SLA parts are susceptible to shrinkage and distortion even afterpost-processing. Heat, moisture, and contact with chemical agents andstrong solvents will affect the color, shape, and integrity of thematerial. Moisture and heat causes the part to soften and creep, whilecontinuous exposure to UV light will increase the opacity of the resin.These wavelengths already cure the resin in the build chamber, andoverexposure to UV light will embrittle the parts.

The table below presents advantages and disadvantages of SLA.

Main Advantages: Main Disadvantages: Excellent Surface FinishDegradation from Prolonged UV High Strength Material Properties ExposureAvailability of Transparent Post-Processing Requires HAZMAT MaterialsPost-Curing UV Process Required High Build Speed LimitedBiocompatability for Low, Predictable Shrinkage factors ProlongedContact for resins

Extrusion Based—Fused Deposition Modeling

Fused Deposition Modeling (FDM) creates layers by extruding beads ofmolten thermoplastic which bond as they contact the part surface andimmediately cool. FDM can utilize many compositions of plastic—the mostcommon being ABS, Polycarbonate, or a combination. New variations andcombinations of extrusion head design are appearing quickly, but themost common is one nozzle for support material, one nozzle for buildmaterial. The build chamber is a heated space, maintained at atemperature just below the material's melting point.

Within this heated environment when one layer of liquid plastic contactsthe semi-molten layer beneath it they will harden together as the twolayers bind. Build and support material is feed in like spools, andafter the extruder has completed the cross-section in the X-Y plane, theplatform drops one layer thickness for the next profile. Thermoplasticmodeling material feeds into temperature controlled FDM extrusion head,heated to a semi-liquid state. The resulting part will be anisotropic,with structural properties dependent on XYZ direction, and will have arelatively coarse surface finish. See FIG. 7 for an illustration of anFDM setup.

Post-processing for FDM requires removing the support material, which iseither broken away manually or washed off using soap and water in anultrasonic bath. The latter uses support materials which arewater-soluble (WaterWorks-soluble support system). For internalstructures, it is near impossible to remove the breakaway supportconfiguration, but if only built externally, readily separate when thepart surface is flexed.

The Z-height layer thickness ranges from 0.15 mm to 0.35 mm from a wirefilament typically 1.15 mm in diameter. The high viscosity of theplastic limits the deposition rate, and resulting build speed since theentire cross section must be filled with material. The smallest featuresfor an FDM cross-section are limited to twice the diameter of theextruded bead because it will always trace an outline of each edge forthe cross-sections before filling in between. The table below presentsadvantages and disadvantages of FDM.

Main Advantages: Main Disadvantages: Minimal Waste of build MaterialUnpredictable Shrinkage Ease & Simplicity of Delamination Rate impactsPost-Processing material properties Ease of Material Change in BuildInexpensive Material

Multi-Polymer Jetting

Multi-Polymer Jetting (MPJ) systems (e.g., from Objet) have thecapability to manufacture parts out of multiple materials within thesame part or platform. The technology is based on hardware fromtraditional inkjet printers, but deposits rows of material that eachhave thickness. The photosensitive ink is immediately cured by a UVlamp, and kept flat with a planar; which both trail the build stage. Itis compatible with a wide range of materials with different properties,and can produce rubber-like parts with various Shore A values. FIG. 8illustrates a multi-polymer jetting process. This fabrication processhas capabilities to produce thin heterogeneous structures, with discretefeatures possessing different mechanical stiffness values.

Each region is saved as a separate STL file, and are aligned by using acommon coordinate system. This allows multiple features to be connectedwhile being built with different properties. The technology is commonlyused to prototype overmolding, rubber features and coatings, and otherapplications using compliant surfaces. These show that heterogeneousparts of compliant Fullcure 970 TangoBlack and rigid Fullcure 830VeroWhite materials to build membranes with thickness 0.58 mm. The tablebelow presents advantages and disadvantages of MPJ.

Main Advantages: Main Disadvantages: Very high feature resolution CannotBuild Hollow Cavities with Ease & Simplicity of Post-Processing supportmaterial Ability to build heterogeneous parts Sensitive to UV light Highfabrication speed

Powder-Based: Selective Laser Sintering

Selective Laser Sintering (SLS) uses a CO₂ heat-generating laser beam tosinter thermoplastic nylon powder together in consecutive layers to forma complete object. Between building each cross-section, precisionrollers deposit a thin layer of powder on the top of the build chamber.The build chamber is heated near to its sintering temperature and whenthe laser is directed to the profile it heats the particles just beyondtheir melting point and they fuse together. Sintering differs frommelting or fusing because it joins powder particles without deformationscaused by flow of molten material. The narrow beam causes only particlesdirectly in the center of the beam to reach the sintering point andalthough adjacent layers get heated they do not melt and instead serveas continuous support.

The platform descends one layer thickness (range of 0.076 mm) and tracesthe next profile (X-Y plane resolution of 0.178 mm for feature edges).The build chamber is filled with inert Nitrogen gas to maintain aconsistent heat and laser strength until the part is complete. Densityand shape of gathered particles has significant effect on bonding andmechanical properties. Generally with higher density of packing comebetter mechanical properties. After cooling, the powder forms a matrixof approximate density of particle material. Grain boundaries affectmechanical properties like elastic limit and Young's Modulus.Finer-grain materials have higher yield strength and hardness thancoarse-grain.

At high temperatures grain boundaries enhance creep rate in metals,coarse grain is preferred for lower temperatures. It is important tobalance the grain size of the plastic. Large particles will cause thepart surface to be coarse, but if ground too fine then electrostaticcharges can build up and make it difficult to spread powder in 2D layer.This process has been utilized for thermoplastics, composites, ceramics,and various metals. For plastics it is commonly some derivative ofpolyamide (nylon, sometimes glass-filled), and for metals is commonly acombination of titanium and stainless steel. FIG. 9 illustrates anexemplary SLS system.

For as many hours as it takes to build a part, it is required to cooldown the chamber before the part(s) can be freed from the powder andcleaned. The particles neighboring the part walls stick slightly to thefinished part from thermal conductivity. As part of the post-processing,these particles will need to be blown away with compressed air andsometimes sanded.

Materials have a common quantified shrinkage of 3-4%. Combined withresidual stresses from thermal bonding and cooling it increases thetendency for thin parts to curl, bow, or warp if they are improperlystored before cooling completely. The un-sintered powder from a buildcannot all be reused for new parts since it has deformed. Afterremaining in a prolonged state of elevated temperature it will not bindas predictably as before. This necessitates using at least 40% virgin(i.e., not previously heated) material for every build platform. Theremaining powder from previous build cycles is considered scrap anddisposed of or recycled. The table below presents advantages anddisadvantages of SLS.

Main Advantages: Main Disadvantages: No Support Structure RequiredAlmost 50% Virgin Material per Materials Available for High PlatformFlexibility High Startup Power Consumption Biocompatible MaterialsAvailable Cost-Effective Build Requires Filling for Short TermImplantation, and an Entire Chamber sterilization Large Infrastructurerequired Actual Nylon as time-stable material

Embedded Electronics

For more extensive monitoring and greater traceability of a wearer'smedical state, electronic sensing and data transmission components maybe embedded into devices either worn or in close proximity to the body.This allows for iterating design and geometry changes as necessary basedon one or a combination of patient feedback, biomechanical analysis ofthe device and its wearer, and measurements taken by embedded sensingelements.

Various models have been discussed for process planning of embeddingelectronics occurring anywhere between the design to post-fabricationstages of device creation. Customizable sensors which can interact witha human at a meso scale without being overly complex pose unique designand implementation challenges. Two in particular are integration withthe surface geometry and device body, as well as the range of compliantand rigid mechanical properties of the soft tissue. A number ofdifferent types of electronics and sensors have been embedded intodevices for the purpose of sensing physical phenomenon, or transmittingits data.

Piezo-Resistive Sensors

Piezoresistivity is a material property where the electrical resistancechanges from an applied strain. The internal arrangement of the atoms'energy bands dictates the degree of this effect. Metals andsemiconductors both have piezoresistive properties, and even insulatingmaterials are able to be endowed with this characteristic by doping themwith conductive particles. The size, shape, concentration, and dopingmaterial itself all affect the degree of this effect. Flexibleinsulating materials like rubber or foam can be made conductive by usingthe doping The piezoresistive phenomenon differs from the piezoelectriceffect in that strain induces a change in electrical resistance only,whereas the latter produces an electric potential.

From theory of mechanics, the direction of the applied strain isimportant since the longitudinal and transverse strains will differslightly according to the Poisson's ratio. Likewise, the respectivepiezo-coefficients will slightly differ, even if the material isisotropic because the cross section shrinks. In a semiconductor thedominant value is associated with the dominant stress. The unitresistivity (also referred to as volume resistivity to note that currentpasses through the material, not along its surface).

Strain Gages

A strain gage is a thin metal foil of a single lead, arranged in arectangular zig zag pattern to measure small deflections at theapplication surface. The metal foil is a piezoresistive material on aplastic backing designed for uniaxial strain sensing. FIG. 10illustrates an exemplary strain gauge. They can be arranged in acircular array to form a rosette, each one using a Wheatstone bridge toconvert the resistance to a voltage and measure it according to a knownreference. They can be delicate to apply and susceptible to thermaldrift.

FIGS. 11A and 11B present different research and commercially availablesensor techniques that use the piezoresistive effect for humanbiomechanics sensing.

Sensors Based on Infrared and Optics

Infrared (IR) sensors use a photo diode to determine the wavelength andintensity of light as outputted by an analogue signal. These devices canrecord the environmental conditions using just this component, or whenin a controlled chamber use a light emitter as the known value and becalibrated when the pathway is interrupted or partially occluded. FIGS.12A and 12B present different research and commercially available sensortechniques that use IR/optical for human biomechanics sensing.

Sensors Based on Magnetics

Using the hall-effect to measure proximity of permanent magnets is apopular choice for robust two-state sensors. In many automobile modelsfor example, the ignition key turns on the engine using a magnet andhall-effect switch.

Using a rectangular array of four hall-effect sensors around a permanentmagnet, a modular sensor has been embedded in polyurethane. Multiplearrays of this design have been applied to the end effector of a robot(e.g., the ‘Obrero’ robot) as a sensing skin. The compliant design waschosen to mimic the performance of human skin, and to overcome some ofthe challenges the researchers had previously found using FSRtechnology.

Sensors Based on Capacitance

Design of a capacitive-based force sensor array have been developed byresearchers at the National Taiwan University in Taipei that can senseboth normal forces and shear forces by detecting the displacement of aflexible polydimethlysiloxane (PDMS) membrane. Approaching the designfrom a MEMS standpoint, Cheng et al. use PDMS as the flexure, and anarray of four capacitors as the strain transducers. The components ofnormal and shear forces are determined by the magnitude and distributionamongst the four sensing elements under the dome. Similarly to themagnet & Hall effect design in the previous section, this uses aninexpensive rectangular array of OEM sensors to create a 3-axis sensingdome. The foot print of each sensor is 8 mm×8 mm.

Sensors Based on Conductive Materials

Conductive materials can act like wires, radio antennae, or contactswitches. The composition of these traces can be from simple graphitesuspensions to more rare materials like platinum which are highlyconductive. The methods of deposition on the exterior of surfaces rangefrom extrusion to aerosol jetting, similar to airbrushing. Thesetechnologies for creating for conformal electronics are sometimesreferred to as ‘direct print’ or ‘direct write’. FIGS. 13A and 13Bpresent different research and commercially available sensor techniquesthat use conductive materials for human biomechanics sensing.

Force Sensing Resistors

The technology for Force Sensing Resistors (FSR) uses degree of contactbetween two thin surfaces to measure how much force is being applied.Although at first this may seem like a piezoresistive effect, andoperationally they are very similar to strain gages, but the compositionof the films are contact based because they are intended to have aconstant unit resistance. Although termed to detect force, an equallyapt name would be ‘pressure sensitive resistor’ since the measurementdepends on a load applied across the circular detection area. Dependingon the composition, they can measure forces up to 120 lbf, acting like avariable resistor having a range for example from 0 to 1.2 kΩ.

The sensor is constructed of three regions: (a) the base active areawith two electrodes leading out; (b) a spacer ring on top along theperimeter; and (c) an application disc with conductive ink screenprinted on the underside. As the applied force within the sensing areais increased, the amount of conductive ink connecting the two electrodeson the base area increases, and the resistance of the sensor drops. Thethickness of the spacer is typically between 0.03 mm and 0.1 mm and maybe screen printed of a pressure sensitive adhesive, may be cut from afilm pressure sensitive adhesive, or may be built up using anycombination of materials that can both separate and adhere to the twosubstrates. Some variations have a third conductive layer orhigh-temperature materials, but hold to the same general operatingprinciple. Some major advantages of industrial FSRs are their low cost,thin profile, and flexible substrate. Some disadvantages are theconditioning requirements, sensitivity to surface area of loadapplication, drift, and hysteresis compared to high-accuracy strain-gagebased load cells.

Shape Deposition Manufacturing

Shape Deposition is a manufacturing paradigm which incorporates theadvantages of several processes including Additive Manufacturing, 5-axisCNC machining, shot-peening surfaces for stress relief and‘microcasting’. Between the stations for these processes, a robotizedpalette can move a part job, and allow fitting of other components likecircuit boards and mechanisms in between the stages as shown in FIG.14A.

SDM techniques have allowed parts to be built with electronic componentsalready assembled inside the body of the part for sensing and actuation.Researchers in SDM advocate that in fabrication of small scale robotics,the fasteners can dominate much of the complexity, volume, and mass. Oneof the aims of this process is to remove these constraints. This alsoallows for more biologically inspired design, as in the case ofintegrating electrical and mechanical components for the robotic hexapodshown in FIG. 14B. This figure illustrates SDM stages of Robotic InsectBody with embedded components [ref Clark 2001 from Stanford].

As shown above, the geometry cavity was milled from wax, the componentswere inserted, and casting material was poured to fill the voids, withone final milling operation to clean the exterior surfaces. Havingaccess to the interior of the part during the build enables placingpassively compliant components by embedding material sections of varyingstiffness. In design of a compliant under actuated hand, flexiblematerials were inserted during the fabrication process to remove theneed for fasteners

The entire SDM process uses several high-resolution machines whichincurs combined time and cost overhead. One version of the shapingstation used was a 5-axis CNC milling machine with a 21-head automatictool changing mechanism. SDM is a highly flexible set of processes, andavoids many of the challenges associated with using conventional AMmaterials. However it is currently still a specialized research processnot readily available to commercial sources like on-line vendors.

Small Scale Mechatronics

The ability to embed specialized components and geometries insmall-scale mechatronic devices is possible through techniques otherthan SDM. FIG. 15 presents different research and commercially availablesensor techniques that use small scale mechatronics for humanbiomechanics sensing.

The described embodiments provide a methodology that integrates sensordesign with device architecture for equipment which interacts with thehuman body. The objective of embedding sensors into custom devices maybe achieved, for example, by using an Additive Manufacturing (AM)approach. Although a variety of fabrication methods may also be used forthe described embodiments, the group of AM technologies are shown in theexamples herein because they provide the most flexibility and agility ofresources for a small customized group of devices. Additionally, AM hasa unique fabrication ability to create parts with voids and cavitiesinside whose geometry has high resolution, accuracy, and repeatability.

The interior sensing element is relatively similar for each type ofsensor (e.g., force, torque, impact) but the surround flexure changesdepending on what phenomenon is desired to detect. The mechanicalstiffness of this flexure is dictated by the surrounding geometry, whichis customizable according to the magnitude of force, moment, etc. Thisbeing said, the sensor can be designed to suit low frequency & largemagnitude strains of for example lower extremity orthotics, highfrequency, lower magnitude strains for upper extremity devices, orsomething in between.

By specially designing the geometry of voids and cavities of a device,and injecting into them a conductive elastic gel, a low-cost designoption for interaction sensing as well as device self-diagnostics ispossible. Depending on the design of the surrounding structure manydifferent types of mechanical sensor are possible, and standalone sensorexamples are developed in this chapter for force, torque, and impact.

The possible applications of an embedded sensor have a wide range ofrequirements which may even sometimes conflict. For the exemplarymodalities in this chapter, the sensor requirements will be selected asif the described embodiments are used for upper extremity biomechanicsmeasurement. For human-interaction studies, force sensing resistors(FSR) would normally be the first low-cost choice. The describedembodiments lower the barrier to adding instrumentation by approximatingthe performance of comparable FSR technology but allowing greater designflexibility, ease, and cost.

The fabrication options with the AM fields were compared according torange of physical build capabilities, as well as the material selectionsand unique attributes. Transparent interaction with the wearer was themost important requirement for the interface. The presence of thesensors should not alter the user/wearer's actions from discomfort, andshould not pose a danger or hazard to them. Lastly, for commercialforce-sensing resistors the lead time for custom shape and loading rangecan take between 2-4 weeks, or longer in case of high demand. In someembodiments described herein, the entire CAD, fabrication, andpreparation for a sensor device may take less than one week. A set ofsensor requirements associated with the described embodiments iscompiled in four categories in the table below to compare to commercialalternatives.

Sensor Goals to Match FSR Technology for Upper Extremities AspectRequirements Specifications Force Accuracy Linearity = 4% SensingLongevity Drift <5% log time scale Performance Low-Medium Force RangeForce range up for 20N Hysteresis Hysteresis 5% Full Scale Outputresistance measurable Low cost additional amplification hardware Singleaxis sensing Examine effect of shear forces Number of cyclesfunctionality Examine up to 5000 cycles Fabrication Does not require AMhardware Existing AM machines are candidates for modifications for SLA,FDM, SLS, fabrication MPJ machines Minimum Build Feature Resolution 0.5mm Hollow cavity & void capabilities Materials have time-stableProperties Flexure does not mechanically or chemically degrade over timeLow-cost Batch device quantity is under $10 per sensor Human Comfortableagainst the body No sharp protruding features, smooth Interactionintegration with surrounding structure Functional for range of body Canbe used against boney protuberances and compliances musculature tissuerigidities Non-intrusive Can be built as near imperceptible to the userPoses no serious biohazards pre or In superficial contact or vesselrupture non post-cure toxic Customization Short Lead Time for fullcustom is under 1 week Minimize operations to customize & Design andembedding process has foresight embed sensor into design for automationEmbedding process has high agility Embedding process does not change foreach device Each sensor individual customization Geometry issensor-specific and location- independent Inputs taken from user,designer, and their data

While the exemplary embodiment utilize piezoresistance, other sensingphenomenon such as magnetoresistance, capacitance, and inductance mayalso be used, although with varying degrees of effectiveness.

For example, capacitive-based sensors such as dielectric polymers can behighly accurate and cover large surface areas. They use the contactbetween two thin films to measure the buildup and passing of electronsto relate back to contact and sometimes pressure. The specializedgeometry to provide the deflection ranges between the films can beseveral orders of magnitude lower than most AM processes. However, thepractical challenges of inserting or attaching two thin film materialsover a variety of non-planar geometry may be more complex than using anexisting off the shelf solution to assemble into a cavity of the device.

The magnetoresistive and ferromagnetic phenomenon are viable as optionsas suspensions of ferrous particles which can be injected and thenmagnetized in hollow cavities of the device. Either method may induce anelectrical change in the presence of the field of a permanent magnet andcould be measured either with a Hall Effect sensor, or as a variableresister. The process of producing permanent magnets requiresspecialized equipment to polarize the ferrous particles and needs to bein a specific orientation and alignment. This may not be as practical asother sensor phenomena for unique custom parts with non-regularly placedsensors in varying orientations.

Conductive liquid suspensions which remain in a liquid state tend tosettle if not rotated periodically. The liquid channels themselves mayalso be susceptible to leaks and require a flexible reservoir adjacentto the sensing site. Being able to fit the sensing material into thecavities necessitated it to be in an injectable/extrudable gel orfluidic state during the build and then later solidify to retain arobust shape.

Accordingly, some of the described embodiments may utilize a materialfrom the family of piezoresistive elastomeric suspensions would offer aneffective sensing element to fulfill the specifications.

Sensors according to the described embodiments work as a transducer thatconverts mechanical deformations to detectable changes in electricalsignals. The core of the sensor element takes advantage of apiezo-resistive polymer within an AM structure that is integratedseamlessly with the surrounding device body. The mechanical propertiesof the AM material and the physical properties of the geometrysurrounding the sensing polymer dictate the mode and amount of strain itwill undergo. Specific geometry can limit deflection to a single plane,while the material stiffness and elastic range dictates the physicaldeflection. This can be controlled by selecting dimensional propertiesin synchrony with the build material so that the flexure's maximumelastic deformation is always selected for the anticipated loadingrange.

Commercial force sensors have a sensing element which is strained by thedeformation of a well-characterized reasonably rigid exterior housing.Such commercial versions however cannot be embedded within the body ofsmall devices and are complexity and cost-prohibitive for manyhuman-sensing applications. In addition, to act as a self-diagnostic ofthe mechanical fatigue and performance of the medical device, thesecommercial structures could not be distributed throughout the volume ofthe device.

Instead of aluminum or stainless steel as the spring element to strainthe sensing component, some of the described embodiments use a springflexure element built from AM materials as shown in FIG. 16. Thepiezoresistive polymer is contained in the polymer bridge 1602 and actsas the variable resistor element in the sensing circuit. An electrode1604 at either end of the polymer bridge 1602 is the connection toattach the sensor to a circuit. Except for the volume of the sensingelement, the geometry and dimensions of the polymer bridge remainsindependent from the device geometry surrounding the embedding site. Thedevice design can be adapted from a legacy part or taken from a 3D scancontaining freeform geometry like that of the human body.

The range of geometry for the polymer bridge and sensor may beconcurrently based on structural characteristics, as well as electricaland fabrication capabilities. The dimensional design may be iterative asmore constraints and benefits are determined from the polymer to beembedded, the polymer electrical properties, and sensor designrobustness. The bridge 1602 may have a constant cross-section (i.e.,constant along the length of the bridge) for homogeneous flow ofelectrons and to avoid geometries which create sudden pressure step whenthe polymer is injected, although other cross sections may be used. Oneembodiment of the bridge may have a circular cross section (althoughother shaped cross sections may be used) because the correspondingradial symmetry avoids shear friction concentration areas when thepolymer is injected, and because the bridge itself will have the higheststiffness in each axis. The circular profile also simplifies the CADprocess because it maintains a constant depth profile, which makes thebridge and injection line immune to rotational alignment challenges thatcan result from creating 3D swept cut features in the device volume.This is also why the polymer bridge may have a constant wall thicknessaround its central axis to contain the conductive material.

Repeatability and reliability of the signals is tied to the linearity ofthe flexure deflection. Thus, the described embodiments may constraindeformation to remain within the elastic zone of the stress-strainprofile of both the conductive sensing material and its AM flexurehousing. This is assuming the selected sensor function is to measureforce/strain from ongoing use. For a safety warning in case of a deviceover-strain, it is only necessary to measure once a strain which exceedsthe proportional limit and alert the user.

The mechanics of the polymer bridge dictate the range of forces theflexure of the sensor can elastically experience. The instantaneouslongitudinal strain of the polymer dictates its electrical state and iskey to simulating its electrical response. Its significance is thedegree to which the conductive particles are separated. FIG. 17Aillustrates the general working principal for piezoresistive elastomersuspensions.

Mechanical deformation considerations exist for exploiting thepiezoresistive longitudinal strain properties. In the case of a simpleand well-characterized geometry such as a cantilevered beam with a pointor distributed load, the neutral axis passes through the centroid of thebeam which creates a zone for tension and a zone for compression. A beamin pure bending has no axial stress on it so wouldn't have any strain atthe particles on the neutral axis. Piezoresistive materials operate on apremise of uniform tension through their cross-section to pull theconductive particles away from each other. Having a compression zoneadjacent to the tension zone could have unpredictable electricalresponses.

One option to retain a cantilever design is to introduce the hollowpolymer bridge area to the tension side of the beam and keep it as acantilever. However this design can be quite fragile for anything butvery small loads and is sensitive to off-planar loads. Clamping thebridge on either end as a ‘built-in beam’ design ensures longitudinalstrain along the centroid of the bridge, keeps the radial symmetry, andoverall stiffens the sensor geometry. Assumptions for this modelinclude:

-   -   Silicone is not structural, i.e. it will not have any mechanical        support.    -   Silicone will not have any longitudinal slip relative to the ABS        hollow bridge, i.e., the strain of the flexure is the strain of        the sensing element.    -   There is planar XY stress only.

Since both ends of the bridge are clamped, each side has two reactionforces and a moment, making the problem statically indeterminate sincewe have six unknowns and only three equilibrium equations. However wecan remove some by using symmetry conditions since the load is applieddirectly in the center of the beam. The deflection equation for thebuilt-in beam is a 4^(th)-order equation with a 2^(nd) order spatialderivative along the beam axis. Maximum deflection occurs at the beamcenter, along the line of symmetry. This value dictates the magnitude oflongitudinal strain. When designing the polymer bridge of each sensorfor its application device, the force will be the primary specificationin the analysis, followed by the three geometric parameters: tube innerdiameter, tube outer diameter, and bridge length.

It can be shown that in the case of a beam in simple bending, thelongitudinal strain along any plane parallel to the deformed neutralaxis is constant. This is not valid for the conditions here because thedeflection profile is a curve with non-constant radius, due to theclamped conditions at either beam end. The strain will vary along thebridge with minimum values at the boundary edges &center point since theslope is zero, and maximum values at x=L/4, and x=3L/4 from maximumslope and the symmetry conditions. Therefore it is helpful to examinethe strain plot of the entire beam and check the average strain whichthe polymer is subjected to.

Exterior: AM Housing Special Considerations

The particular geometry of the described embodiments, as well as thescalable design requires very specific fabrication capabilities whichcan also easily produce a wide range of sensors; both stand-alone andembedded in the body of a device itself. This makes using AdditiveManufacturing (AM—also commonly referred to as ‘Rapid Prototyping’ or‘layered manufacturing’) attractive, which can be used to build highlyaccurate features at a small scale, including hollow features and voids,thin surfaces, and is expandable for a mass customization platform witha variety of materials. For medical sensing applications, although it isimportant to select parts based primarily on material properties, it isnot mutually exclusive from its AM fabrication process. They areexamined concurrently since the process settings will significantlychange the material properties expressed in a part.

Enabling Solvent Escape

The AM material structure around the polymer bridge needs to offerappropriate conditions to completely cure the conductive material. Inthe case of solvent based epoxies and silicones, there needs to be someway of allowing the solvent gas to escape the silicone during thedegassing phase. The solvent can depart from the polymer via pores andmicro cavities in the AM material, or by chemically combining with it.Examples of these two methods were evaluated using samples of SLS nylon12 and SLA Accura 40 plastics, which were imaged with a scanningelectron microscope for micro pore structure to evaluate solvent escape.

The nylon used in the SLS process is chemically non-reactive to thesolvents but contains a pore structure that allows the gas to escape.Completeness of curing is confirmed by the resting resistance value of apolymer bridge.

During the sintering process, the core regions of a part will have beenkept at an elevated temperature for longer since there is a higher laserdwell time, as oppose to the edges where it dwells for the shortest timeand the particles between layers don't fuse together as completely. Asthe depth of the layer increases, each layer becomes more solid andporosity reduces, which is more challenging for the gas to escape into.Therefore for best possible cure conditions the sensor site andinsertion channels should be closer to the surface/outer edges of thepart features.

The SLA parts are non-porous and have the most well-ordered crystalstructure, but to a certain degree still allows curing by chemicallyaccepting/combining with the solvent up to a saturation point. Thismaterial exhibits the conditions most challenging for the solvent toescape because the threshold is material-dependent, and it maystructurally degrade the bridge interior. The SLA parts have no porestructure or room for the solvent to escape into. It has been confirmedthat the polymer reaches full cure under certain geometric conditions,and a portion of the channel interior was dissolved. The SLA resin ischemically sensitive to strong solvents like isopropanol or ethers evenin its fully cured state, which reinforces the assumption that thesolvent gas is reacting with the polymer bridge interior. Although thedegraded tube still conducts electricity, the degradation creates a deadzone and reduces sensitivity of any strain-based sensor design becausethe silicone is no longer strained equally to the SLA flexure around it.Cross sections of the ABS FDM and SLS Nylon 12 tubes did not exhibit anysimilar reactions.

Orthotropic Mechanical Properties

Mechanical properties of AM fabrication materials are orthotropic bynature and are dictated by their build orientation. At least some of theembodiments measure the strain state of the surrounding material in theelastic zone and notify the user of the forces without crossing theyield point into permanent deformation. Depending on the sensorinsertion location, the orthotropic material properties will affect thestrain limit. SLS nylon is an ideal AM method for building devices usingthis solvent-based elastomer sensor because the porosity can bewell-controlled by the sintering parameters, in addition to having ahigh elastic limit and being biocompatible.

Results were averaged for five samples built in three orthogonal Z-axes.It is also worth noting the difference in failure mode depending onorientation. Sample A acted more like a brittle SLA material since itdid not have a well-defined yield point with any plastic deformation. Aplastic deformation zone is also an important safety consideration tominimize hazards to the wearer in the event that a failure modeoccurred.

The part will always carry the highest mechanical properties when theZ-build axis is normal to the cross-sectional build planes of greatestsurface area. Sample B had the greatest ‘necking’ and largest elasticdeformation zone. Sample C was more prone to failure at stressconcentration zones, and sample A was the most brittle. If orientationof sensor vs build/part orientation is an influence on performance thenthe robustness of measurement will need to be verified and accounted forin signal acquisition or filtering.

Interior: Conductive Material

Sensors of the described embodiment include a conductive elastomer withpiezoresistive properties. A suitable material exhibits a change inelectrical properties when subjected to strain from the AM structuresurrounding it, while complying with the insertion limitations fromoperating within hollow cavities and be safe to use alongside humans.Polymer requirements may be subdivided into four categories based onphysical properties, and ease of integration into the AM polymer bridgedesign.

Integration with Current Vendor Processes:

To facilitate commercial accessibility the end AM components should bebuildable in nearly any service bureau, and either the end user caninsert the conductive elements themselves or the bureaus can easily addit to their service capabilities without modifying their current AMhardware to embed the conductive material. This also necessitates theconductive material itself to be readily available and not require anyspecial handling procedures outside of good laboratory practices. ShapeDeposition Modeling (SDM) is a technology specially suited to insertcomponents and materials during the build phase, but requiresspecialized hardware and the technology are currently not widelyaccessible to researchers or consumers via service bureaus.

Insertion [Embedding] & Preparation of Conductive Material:

The sensor operation site may not be adjacent to the insertion site, sothe material had to be able to pass into the sensor site withoutdeteriorating. If the conductive material was inserted in an uncuredstate, it had to cure within the partially enclosed chamber for thesensor, in conditions of ambient temperature and pressure. Many AMmaterials have creep temperatures below 200° C., which is a commoncuring temperature for compression molding to vulcanize elastomers. Mostpiezoelectric and ferroelectric materials require deposition andcrystallization conditions for temperature ranges of 200-800° C. The AMstructures may also have delicate features and it would be challengingto pressurizing the chamber containing the conductive material. Thisexcluded many silicone rubber suspensions since they require highpressure and temperature to cure, and AM thermoplastics as well as manySLA resins have creep temperatures far below the rubber curingtemperature. For manual insertion there is a practical expectation tohave a few minutes between removing the conductive material from itscontainer and inserting it into the cavity of the AM structure. Theinsertion requirement is specified as a pot life of at least 5 minutes.

Electrical Properties:

The target electrical properties are a combination of performingfunctionally like a strain gage, with some common characteristics ofpotentiometers and FSR.

In a sensing resistor, current is being spent as thermal energy throughthe resisting element. A high electrical impedance (many FSR elementsare in the MΩ range) would reduce the amount of current moving throughthe sensor at the resting state. This also minimizes the susceptibilityto thermal effects and maximizes battery life when self-powered. FSRsare usually accompanied by a relatively high change in resistance duringloaded (0-50% operational range). Comparatively, typical strain gageresistances range on the order of 0.01-1 kΩ. Although sub ohm changes inelectrical resistance can be measured and amplified it was preferable tosee changes in at least the Ω range when the material is strained themaximum anticipated distance of 2-3 mm.

Ohm's Law between two measured points in a conductor

V=IR

where:

-   -   V=potential difference between measured points    -   I=electrical current, A    -   R=resistance between measured points, Ω

Time-varying effects like capacitance should be avoided. For a chargebuildup or dissipation it can affect design of analogue & digitalfilters, and would become a limiting factor for the maximum measurablefrequency.

Mechanical Properties:

The mechanical properties of the internal sensing material shouldideally not limit the mechanical structure but act ‘invisibly’. Tominimize complexity of modeling, the effect on mechanical stiffnessshould be negligible, while sensing the entire elastic range ifcontinuing operation is required, or some of the plastic region ifone-time failure warning is required. Linked with the mechanicalrequirements via the Gage Factor, the material should be able toelectrically sense throughout its own elastic range to maximize thelevel of operational strain.

The requirements from the preceding categories are summarized in thetable below.

Polymer Requirements Category Inclusion Goals Exclusion CriteriaIntegration Commercially available in High toxicity pre or with varietyof volumes post-cure Vendor Does not require AM Cost prohibitive forCapabilities hardware modifications $10 per sensor Shelf life is under 6months Insertion and Can be inserted into hollow High insertion pressurePreparation cavities post-build requires complex hardware Vulcanizesunder room temp, Low viscosity prevents pressure, humidity even curingAble to cure within hollow Destabilizes surrounding cavity of polymerchannel AM material Electrical Consistent volumetric Time-dependentproperties Properties resistivity Stable for Time, temperature, andhumidity Linear gage factor Inconsistent gage factor Sensing range isequal to Electrical range is binary mechanical elastic range stateMechanical Elastic limit greater than Material is brittle Properties AMplastics Material is more compliant than AM plastics Stable for Time,temperature, Significant shrinkage after and humidity cure Particlesremain in suspended High Poisson's ratio causes positions, don't settleseparation from polymer bridge interior

While a variety of commercially-available conductive materials may beused in the described embodiments, the materials summarized in the tablebelow provide an example of suitable materials. Many are conductiveadhesives which are commonly used for electrically grounding enclosuresor repairing broken electrical connections.

Operating Application Sample Curing Material Principle Properties MethodResistance # Hazards Conditions Silver Paint, Silver particles Fastdrying, Well Shaken in NA 1 Inhalation Needs 18% Silver in evaporatingconductive syringe when uncured, complete air Silver Paint, suspensionWell Shaken in NA 2 flaking when contact for 50% Silver syringe curedcuring Scotch-Weld 3 min work 2 part applicator NA 3 Eye, skin, RoomDP-100 Clear life plunger respiratory Temperature Adhesive irritant andPressure Scotch-Weld 4 min work 2 part applicator NA 4 Room DP-100 lifeplunger Temperature PLUS Clear and Pressure Adhesive Conductive Silver-Pre-packed  31 Ω/cm 5 Ground Needs Flexible suspension for applicatorpen Shipping only complete air Circuit Pen conductivity b/c of toxiccontact for Hazard curing ResinLab Silver particles 40% strain 1 SyringeNA 6 Severe Room Silver densely packed hr pot life, inhalationTemperature Conductive $26 hazard and Pressure Epoxy Conductive Copperand tin resistance Thread through 2.7 Ω/cm 7 None None Thread woven intochanges for cavity thread, strain thread increases tension resistancePure Silver High $28/50 g Syringe NA 8 None Room Conductive ConductivityTemperature Epoxy and Pressure Silver High Single Part Syringe NA 9 Eyeirritant Room Conductive Conductivity Temperature Grease and PressureSilver High Single Part Syringe NA 1 1 Eye irritant Room ConductiveConductivity Temperature Grease and Pressure 2 Part Conductivity Alreadyin Syringe NA 10 None Room Conductive Syringe, high Temperature Epoxyconductivity and Pressure SPI Conductive High Cured film is a NA NASevere Room Conductive Silver particle conductivity, compositeInhalation Temperature Silver Paste suspension absence of consisting ofhazard from and Pressure wicking flakes of silver CO₂ & colloid in aalcohol. polymer matrix Explosive Elastosil LR PiezoresistiveElectrically Industrial Press  11 Ω/cm NA NA 10 min at 3162 A/Bconductive, 165C in press Shore 51A, 400% elongation,

The sections below describe a process for one or more of a large groupof candidates for a conductive material according to the describedembodiments. It should be understood, however, that such narrowing isnot meant to exclude a candidate for use with the described embodiment,but rather to select a particular one from a larger group of suitablematerials.

Each candidate was injected into a simplified bubble test specimen tosimulate the instrumentation process and the candidates were narrowedaccording to the criteria described above. Each bubble test had 10samples of the material and was checked for conductivity after thespecified curing time. The dimensions of the bubble test for length andinterior diameter were chosen from the easiest (largest) geometry goalsof the final sensor.

The bubble test was an opportunity to compare the ease of preparing arudimentary polymer bridge of the described embodiments. Most of thematerial samples could be injected via syringe, the translucent Accura40 resin allowed observation of voids or cracks appearing in the curedstates. Copper electrodes 0.016″ in diameter were first inserted intoeither end of the bubble test chamber to compare ease of filling, curingprocess, contact to the electrode, and conductivity. From evaluationusing the requirements set forth above, all of the candidates wereunsuitable as detailed in the table below.

Sample Material Outcome # Suitability Silver Paint, 18% Silver Volumesignificantly 1 Eliminated shrinks and cracks, non conductive SilverPaint, 50% Silver Volume significantly 2 Eliminated shrinks and cracks,non conductive Scotch-Weld DP-100 Brittle, non conductive 3 EliminatedClear Adhesive Scotch-Weld DP-100 Brittle, non conductive 4 EliminatedPLUS Clear Adhesive Conductive Flexible High toxicity, voids 5Eliminated Circuit Pen appeared ResinLab Silver Too Hard 6 EliminatedConductive Epoxy Conductive Thread Infeasible 7 Eliminatedimplementation Pure Silver Conductive Inconsistent 8 Eliminated EpoxyConductivity Silver Conductive Grease Does not solidify 9 EliminatedSilver Conductive Grease Does not solidify 11 Eliminated 2 PartConductive Epoxy Hard and brittle 10 Eliminated SPI Conductive Silver$130 for 30 g: NA Unavailable Paste Cost prohibitive Elastosil LR 3162A/B Special order size NA Unavailable minimum is 20 L barrel. CostProhibitive

With the commercial options unsuitable, a sample of prepared manually bydoping a solvent-based silicone RTV epoxy with a dense collection ofiron filings. The combination yielded candidate 12, which had poorrepeatability but fit all of the polymer criteria and exhibited aresistance change when bent or strained. With the promising results ofcandidate 12, a suitable commercial version of the same type of materialwas located. This successful material was candidate 13: a silicone RTVsuspension of nickel-coated graphite particles (MMS-020, SiliconeSolutions, Inc., Twinsburg, Ohio). This material met all of therequirements while exhibiting piezoresistive properties and beingcommercially available.

Material 13 is a silicone room-temperature-vulcanizing (RTV) materialcontaining conductive particles of nickel-coated graphite (MMS-020,Silicone Solutions, Twinsburg, Ohio). This material is commerciallyavailable as a flexible electrical insulating material to groundelectronic devices. Although it satisfied the search criteria, none ofthe detailed piezoresistive or mechanical properties was available.

Material 13 is representative of a group of Room Temperature Vulcanizing(RTV) materials which cure by degassing a solvent reaction inhibitor.Common single part solvent-based epoxies include cyanoacrylite instantadhesive “Crazy Glue” and DWP-24 Wood Adhesive “Liquid Nails. When inthe sealed environment of the container, the material remains in aliquid state because the trapped solvent inhibits the curing process.But when applied to a surface, the solvent inside the liquid escapesinto the surrounding atmosphere and the epoxy molecules cross-knit andpull together to form chains. When conductive graphite is suspendedinside this material the end state is that these particles are closeenough together to allow electrons to jump from one to the next whenfitted into a circuit with a voltage differential. Combining thissilicone with graphite adds the piezoresistive response when theparticles are strained apart. Silicon is a good elastomer for thesuspension because it is abundant, inexpensive, and thermally stable.

To facilitate solvent evacuation and speed up the curing processalternative filling procedures were considered. By inserting a needlesyringe down the filling channel and incrementally filling andretreating the needle there is an opportunity for the solvent to escapeback through the filling channel. However this creates boundaryinteractions between the injection volumes and considerably slows downthe filling process. Another option is to vacuum out the solvent throughthe surrounding AM material by creating a negative pressure. Thisprocess would add complexity and for thicker AM structures has a lowlikelihood for success.

Materials like silicone are well known for their electrically insulatingproperties, but these can be modified by the introduction (doping) ofconducting fillers. Current can only flow through the conductors, andthese additions require a minimum concentration to conduct electricitythrough the polymer bridge, and their curing patterns are modeledaccording to percolation theory. Percolation theory is a mathematicalmethodology to understand and make predictions in a continuum ofdisordered media. Each point in the media is defined by a randomvariation in its degree of connectivity to its neighbors. It has beenused to model disordered systems such as spread of disease infections,adoption of social trends, fractals, liquid intrusion into porous rock,and in this case polymerization of chemical bonds. Statisticalpercolation theory predicts for the dependence of conductivity on fillerconcentration as a power law behavior, and percolation threshold refersto the minimum number of connections to create a link between twoopposing ends, called a chemical path. The molecular bonds form as thesolvent is released, and at the threshold join to form the chemicalpath. It is important to note that this is not necessarily the shortestor most electrically efficient path between the two end points; it isjust the first to form. The formation of the chemical path indicates thefirst moment when the polymer bridge is able to conduct electricity. Fewexact results exist since percolation theory describes probabilities.However some results have supported the theory that the shape of thecontinuum plays a role in determining the conductivity and percolationthreshold. If correct, this theory impacts selecting the geometry of thepolymer bridge and injection lines since their length will be thelargest dimension of the continuum.

Values like maximum conductivity and percolation threshold can bebalanced by empirically determining the shape and size of the additiveparticles (dopant). Generally, the closer the volume of the insulatorresembles the conductor, the low the resistance. Larger particles willconduct better because their presence decreases the volume of insulatorthe electrons need to pass through, and when examining the particleshape, flakes conduct better than spheres because of their superior‘stacking action’. In some experiments, the resistance values of fibersand flakes decreased up to 17 orders of magnitude when packed withfiller. Depending on the concentration of the fillers the mechanicalproperties can also be significantly impacted. Graphite is a brittlematerial, and the decreased percent volume of silicone in the mixturesincreases the brittleness of the bulk material.

To examine the deposition, shape, and size ranges of the particles asingle layer was applied to number zero glass slides and allowed tofully cure. Images of the conductive material were gathered usingDifferential Interference Contrast (DIS) Microscopy at 10× magnificationusing a Nikon Eclips TE2000-E camera system. The resulting imagesindicated that the nickel-graphite is a disordered semiconductor

In order to conduct current the particles need to be densely packed,i.e., in direct contact with their neighbors. When the material isstrained, the particles remain attached to the flexible silicon but moveaway from each other and decrease the number of paths for the electronsto travel from one side to the other. The non-uniformity in the shapeand size of the dopant particles all add to variability in theresistance of the samples, especially when the samples are small enoughit may not be representative of the bulk properties.

During the polymerization process, many pathways will form and networkbetween the two electrodes. The micro pathways' combined resistances areequivalent to summing a large number of parallel circuits for the totalresistance of the bridge.

As the polymer is strained the resistance of the pathways will increaseto infinity until only one remains. This single pathway acts similarlyto the chemical path that it may not be the most efficient, but is theonly available conduction route. Since the polymer will always beexamined in a bulk state as a fully cured material, there is nopractical reason to know the resistance of the individual chemicalpaths, just of the resistance of the entire continuum (bridge) atcomplete cure. This bulk electrical property is the resistivity throughthe volume of the material.

A conductivity test was performed to determine the average unitresistivity through the material volume (as oppose to surface area)which estimates resting resistance of the sensor. Using the guidelinesof the ASTM B193 standard, ten samples of uniform length andcross-section were prepared in two sets of tubes. One material wascellulose butyrate, a porous non-reactive plastic to facilitate solventescape; and the second was glass, which constitutes a non-porousmaterial. Tube samples were within 0.003 in length of the 12 in (or 300mm) guideline of the testing standard, and interior diameter was 0.125in (0.3175 cm). Samples were measured periodically for conductivity andlet rest for 30 days to ensure full curing even though the conductivityvalues had settled to 10% of previous measurement after just the sevendays. Resistivity was calculated using the following equation:

$\rho_{v} = {\left( \frac{A}{L} \right)R}$

where:ρ_(v)=volume resistivity, ΩmA=cross-sectional area, m²L=gage length used to determine R, mR=measured resistance, Ω

During the curing period the samples were kept in a temperaturecontrolled fume hood at 20° C. Measurements were collected with a Fluke179 digital multimeter, and results were from an average of 5measurements taken 10 minutes apart after 30 days curing. After the 30day period none of the glass samples had achieved conductivity.

The range of the samples' resistance varies between 7.1 and 84.6 μΩmwith the standard deviation of each sample within 0.03% of its average.The average from the plastic samples is 31.0 μΩm with a standarddeviation of 27.4 μΩm. This variability is primarily comes from the widevariety of particle sizes in the tube batches, but the average is stillrepresentative of bulk material properties. Shape and consistency of theparticles, as well as uniformity of the nickel coating all impact thevariability of resistivity. Individual calibration of each sensor to thedistribution of its particle properties can take these results intoaccount. In addition this range could be reduced with a more consistentand well-controlled process for size and shape to prepare thegraphite-nickel samples. Despite the high standard deviation, theresulting average is consistent with measurements for graphiteresistivity. Resistivity values in this test are higher than those ofpure graphite are because some of the test volume is taken up with aninsulator, which is effectively constricting the electrical flowsimilarly to shrinking the cross-sectional area.

Prototypes from several AM materials were examined for static anddynamic responses in a controlled setting. The sensors and system wasexamined for capacitance and drift while is resting conditions, as wellas responses to various dynamic loading patterns. The resting drift ofthe silicone was examined using the sensor samples in a voltage dividercircuit at 20 degrees Celsius for 45 minutes. A regulated 3.3V powersupply was used with a 100Ω resistor as the other half of the dividercircuit. This first test specimen had a resting resistance of 6.5Ω. Noanalogue or digital filters were used in the circuit.

The data from both samples show small changes on the order of mV,approaching the data acquisition resolution limit. For an expectedresistance change on the order of several ohms, any drift effects appearto be negligible. The small changes in the data may be attributed to thequantization error in both graphs or slight thermal effects in themeasurement space.

The same testing setup was used to check capacitance of the material asit discharges after the power is switched off. A diode in seriesprevented any drain on the samples from the power supply during shutoff. Different frequencies were manually activated. For 40 ms followingeach termination of the power there is a discharge effect. This is aconsideration for the maximum frequency of reliable measurement to avoidinterference from this effect.

The performance of the sensor is dependent on the behavior of theconductive material as the sensing element. Its electrical response andmechanical limits are dictated by four parameters: three geometricdimensions which define the polymer bridge and one from themechanical-electrical relations for gage factor. The following sectiondiscusses the three geometry parameters. The three geometry parametersaffect the polymer's ability to fully cure according to: the volume ofpolymer inside the bridge, and the volume of material through which thesolvent has to escape in order to fully cure. The curing time expectedfor a particular sensor now also indicates how long one can expect towait before a sensorized device is ready to be used and takemeasurements reliably. The impact of these parameters was determinedusing Taguchi Methods.

Taguchi Design's orthogonal arrays were used to optimize parameters forresponse characteristics based on geometry dependent variables. This wayof modeling will yield relationships to determine effects without havingto test a large number of variations of the sensor's polymer bridge.Taguchi methods are strategies based on statistics for the optimizationof an objective function by varying the input parameters. Taguchiintroduced design criteria for robust system tolerances using orthogonalarrays that allow analysis of many factors with minimal trials. Althoughthis method has been used for mass manufacturing for a long time,recently it has been gaining interest for parameter optimization whiledesigning a new part. Taguchi methods have been used in RP processes formulti-variable optimization of output characteristics like surfacefinish, dimensional accuracy, and ultimate tensile strength. Studieshave examines build parameters for temperature, build speed, and builddensity have been varied to determine which output characteristic ismost affected by each input parameter. This setup allows the testing ofall three geometry variables at three parameters without having to run27 (3³) separate experiments.

Based on the overall size goals of the sensor each geometry parameterwas assigned a small, medium, and large value to define the designspace. The three parameters were assigned 3 levels and 5 samples of eachseries were built. Sample series were built using stereolithography(SLA) Acura 40 resin. The chemical interactions during degassing aredependent on the volume to release. Although the SLA resin has a uniquereaction with this solvent, it altogether still represents the mostchallenging conditions for curing since other materials used with FDM,SLS, and MPJ are non-reactive.

When fully cured, the polymer conducts continuously. This is theelectrical indicator of the material state the curing time and curingcompletion is based on the ability of solvent vapor inside the mixtureto escape, therefore different combinations of the geometry wereevaluated to determine ranking of their impact on this degassingprocess.

With the exception of tube length dimensions, the experimental procedureand measurement followed the ASTM B193-02 standard. Each experiment wasfabricated with the tube length aligned in the build Z-axis for maximumconcentricity. This also kept all tube surfaces smooth since no internalsupports were generated. Once the SLA material was post processed andfully cured, the polymer was injected into the tubes using a syringe.Observing through the translucent resin, care was taken to avoid airpockets forming. The syringe was not removed until the tube was filledwith silicone. Samples were kept in a temperature controlled fume hoodat 20° C. Resistance measurements were taken every six hours for aperiod of seven days.

The control condition was the 24 hour curing period recommended by themanufacturer. When fully exposed to the atmosphere, this allows thesolvent to freely escape. The time to first conductivity and the time tostabilize are important values which indicate the ease of the solvent toescape and allow full curing to complete.

As the solvent is released and the molecules cross-knit chains theybecome closer together and allow electrons to move through as asemiconductor. The patterns of the data follow three stages: (1) theelectrical impedance is infinite until the first conductive pathwaycures, albeit constrictive and maintains high impedance; (2) rapid decayof the resistance values is an indication of the number of channeloptions for the electrons increasing to allowing easier flow across thegraphite particles; (3) all of the molecules have cross-knit and theresistance values settle to a steady-state as the silicone is completelycured and the high density of particles allows conduction readily.

The five samples within each experiment followed the same patterns butvaried in their consistency. Experiments 1, 2, 4, 5, and 8 all havecuring patterns that vary, but have similar final resting values. Decayto steady-state values almost completely occurred within 40-50 hours foreach experiment after the first conductive instant. To perform theTaguchi analysis, this time to first conductivity was selected as the‘small as possible’ process objective since it varied across eachexperiment.

The geometry factors impacting curing time in order of significance are:bridge length, wall thickness, and inner diameter. This indicates thatmost of all the length of the polymer bridge should be minimized at 20mm to promote rapid and complete curing of the sensor, with wallthickness and inner diameter also as small as possible, but have adesign window between the small and medium values. Time for theresistance value to settle (stabilize) is the point when consecutive 6hour measurements were within 10% of the previous value for a 12 hourperiod. The Electrical Resistivity has also been calculated and includedin the table to compare the unit electrical conductance state of theexperiment at the end of the 7 day trial. This result is consistent withthe general percolation theory, which estimates the time to chemicalpath is based foremost on length i.e. the length of the bridge has thegreatest impact on time to first and steady-state conductance.

The results from this trial reinforce the conclusion drawn thatminimizing the polymer bridge dimensions decreases curing time. Asexperiment 1 was the first to reach curing state it had the longest timeto settle and stabilized the most, indicated by having the lowestSpecific Electrical Resistance value. It is possible that examining theother experiments for longer than seven days would show similarpatterns, but even if they fully cured with a high repeatability thiswould violate the sensor requirement for having a short lead time of 1week. The samples which did not reach the curing criteria may also havereached the solvent absorption threshold for the SLA structure, and theleftover solvent continued to inhibit the curing reaction.

The average volumetric resistance of the samples here is roughly afactor of 10 higher than the previous volumetric tests. The SLA housingrestricted the solvent release, as well as the length of the experimentterminated 23 days earlier than the volumetric tests. If measurementshad continued for the same full time it is likely that the smallerdimensional samples would approximate the previous average, with thelikelihood decreasing as the Taguchi polymer volume increased since itwill saturate the Accura 40—solvent reaction at some point.

To predict and model the performance of the conductive polymer insidethe force sensor, it is necessary to understand its electrical andmechanical reactions to strain. These reactions were determined fromtesting for Gage Factor (GF) and Poisson's ratio (ν). To determine theGF, mechanical elongation and change in electrical resistance weremeasured simultaneously. Material samples of the conductive siliconewere prepared and tested in a tensile mode to strain them, while thechange in electrical resistance was recorded. Both sets of tensile testswere in accordance with ISO 37:2011(E) standards for molded rubberdogbone samples. The curing conditions of the samples needed toapproximate the interior of the polymer bridge; however a method forproducing repeatable geometry and flat surfaces posed the samechallenges as curing inside the bridge. Several methods were attemptedto make a thin-walled mold to allow the solvent to escape whileretaining the desired shape with smooth surfaces. For larger molds thebarrier at the perimeter of the conductive material would cure first andblock in the solvent gas escaping from the core sections which leavesthe interior uncured. Lost wax negatives and thin walled SLAdouble-chambered molds were some alternative efforts. A substratematerial was required which allowed the solvent to escape withoutleaving a residual texture or crack propagation. Cork, basswood, andseveral porous thermoplastics were unsuccessful since they could not beremoved/cleaned off without gauging and damaging the sample surface.

Foamular 250 extruded polystyrene wall insulation foam (Owens Cornering,Toledo, USA) was successful to act as a secure substrate to support thedogbone surface whilst also allowing the solvent to evacuate. Theopposite surface was contacted by thin polyethylene (0.127 mm) whichreadily peels away leaving a clean surface to apply electrodes tomeasure resistance during tensile testing.

Samples were left in a temperature-controlled environment at 25° C. fora period of 30 days to fully cure. At this point the polyethylene layerwas peeled back and mold alignment tabs were removed. Any samples withcracks, voids, or damage from the de-molding process were discarded.

Ten samples were prepared using the molding methods described above, andtesting was in accordance with ASTM D638-10 standards. Measurements weretaken using an Instron and dual-axis extensometer at 1 mm/min. Resultsfor longitudinal (axial) and transverse strain are shown in below.

This value is a very low Poisson's ratio for an elastic material. Rubberwould normally closer to the theoretic mechanical maximum of between 0.4to 0.5. However comparing the value to the filler material, in thissense it behaves more like concrete graphite who has a ratios between0.1 and 0.2.

The likely explanation is that when the samples are molded or injected,the conductive particles (with very low Poisson's ratio) are compactedtogether inside the chamber, with the micro gaps being filled with thesilicone elastomer (maximum Poisson's ratio). When a longitudinal loadis applied, the strain elongates the silicone along its axis but thereis still very little room for the particles to move closer within thetransverse plane. There is very little transverse strain when the bridgein the sensor is flexed, therefore the polymer will not try to separatefrom the inner cylindrical surface. It is unusual for a flexible/elasticmaterial to have such a low value, but this result is actually verybeneficial because it reinforces the assumption that the strain of theflexure is equal to the strain of the conductive polymer.

The Gage Factor (GF) of a piezoresistive material is the relationshipbetween the change in its electrical impedance (dependent variable) fromchange in its mechanical state (independent variable) of strain. As thename implies, it is usually a single number based off of thelinearly-elastic mechanical deformation (with an assumed linearelectrical changes associated with it). This is the electricalsensitivity of the gage wire responding to strain. The GF is also knownas the piezoresistivity or sensitivity factor, and can be calculatedusing the instantaneous resistance or unit resistivity of the material.The latter takes into account the Poisson's ratio for the shrinkingcross sectional area. It is desirable to have a high GF value because itwill be easier to detect small changes in strain.

When straining a material the cross section perpendicular to the appliedload shrinks. So taking into account using poisson's ratio there isanother option to use a modified GF equation seen below taking this intoaccount. One of the assumptions for the sensor modeling is that thepolymer contained within the bridge does not shift or part with theinterior walls.

Some other materials which exhibit this effect are shown in the tablebelow.

Gage Factor (GF) Ultimate Low High Elongation Material Strain Strain (%)Pure Platinum (Pt 100%) 6.1 2.4 0.4 Metals Platinum-Iridium 5.1 — — and(Pt 95%, Ir 5%) Alloys Platinum-Tungsten 4.0 — — (Pt 92%, W 8%)Isoelastic (Fe 55.5%, Ni 36%, 3.6 — — Cr 8%, Mn 0.5% * Silver (100%) 2.92.4 0.8 Copper (100%) 2.6 2.2 0.5 Constantan/Advance/Copel 2.1 1.9 1.0(Ni 45%, Cu 55%) * Nichrome V 2.1 — — (Ni 80%, Cr 20%) * Karma (Ni 74%,2.0 — — Cr 20%, Al 3%, Fe 3%) * Armour D (Fe 70%, Cr 20%, 2.0 — — Al10%) * Monel (Ni 67%, Cu 33%) * 1.9 — — Gold Paladium 0.9 1.9 0.8 (Au40%, Pd 60%) Manganin (Cu 84%, Mn 12%, 0.47 — — Ni 4%) * Nickel (Ni100%) −12.1 2.7 — Families Metal foil strain gage 2-5 — of Thin-filmmetal  2 — Sensing Single crystal silicon −125 to +200 — Materials Polysilicon ±30 — Thick-film resistors 100 — * Isoelastic, Constantan,Advance, Copel, Nichrome V, Karma, Armour D, Monel, and Manganin are alltrade names belonging to respective owners

Platinum is an excellent conducting material available, widely used insemiconductors and integrated circuits, and also exhibits apiezoresistive effect. Note that of the list of pure metals, nickel hasthe greatest magnitude gage factor which makes it desirable since it isthe most sensitive, however it is negative. Nickel and some other metalshave an unusual GF in that they are strain-dependant, so will firstdecrease resistance for low strain, and then change after a point. Thesenon-constant GF values require a separate model. CIF is affected by thechange in wire length, cross-section area, and the piezo-resistanceeffect of the wire material. The strain sensitivity factor S itselfranges from −12.1 in Nickel up to 6.1 in Platinum. Material-specifictesting is necessary since even between a pure material and an alloy theGF can be quite significant. Graphite is a brittle material so in itspure state doesn't have a GF because its elastic limit is very low.

The conductive silicone was tested for GF in a tensile destructive test.The testing protocol was a combination of ASTM D257-78 and ASTM B193standards for measuring resistivity during tensile elongation of acontrolled volume. Ten samples were prepared for this testing using thepreviously described molding technique. The electrodes were surfacecontacts at opposite ends and sides of the sample to measure thevolumetric, not surface conductivity. The outside of the jaws wereelectrically insulated except through the test sample. All samples weremaintained and tests were run in temperature controlled environment at20 degrees Celsius to avoid thermal changes and effects on the sensingmaterial. A 5V input was used with a Type I quarter Wheatstone bridge,and the signals were post-processed with a 4^(th)-order Butterworthfilter. A first examination of the electrical response to the tensileloading in FIG. 17B shows the failure limit of the samples at 35N and alevel-off for the electrical response. FIG. 17B is the electricalresponse of the ten samples during tensile testing. When the samples nolonger conduct electricity, they have reached the equivalent p_(c) valueas the minimum number of particles in contact to close the circuit.Looking at the resistance of the polymer sample as it is strained, thisis when the material acts more like an insulator than a semiconductor.

The mechanical strain and change in electrical resistivity are plottedin FIG. 17C, which shows that the Gage Factor plot for this material isa non-linear function of strain. The shape of the curve for mechanicalstrain and change in electrical resistivity shows that the dominant GFelectrical response is from the nickel coating. The initial portion as anegative relationship, with a clear inflection point before the positiverelationship is similar to the response of pure nickel. After a secondstrain limit the electrical response levels off, which is the sensingrange of the material, but is still lower than the mechanical limit. Theinflection point was determined from derivative of a curve-fitting, andtwo linear regression zones were overlaid on the sensing range to dividebetween low and high strain.

The GF from the first linear portion is comparable to the expected valuefor nickel in the low-strain state, but the second region is an order ofmagnitude higher, which is a greater sensitivity to mechanical changes.The second zone was cut off at strain of 0.00863 where the responselevels off. This is considered the strain limit for electrical sensing.The gage factor itself was plotted versus strain with the two linearportion overlaid. The variability in the GF from the 10 samples affectsthe accuracy and repeatability of the sensor once the polymer will becontained within the bridge.

The two electrical response zones were overlaid upon the mechanicalresponse curve. The negative GF region was found to be coincident withthe non-linear mechanical zone and the positive GF was found to becoincident with a linear mechanical region.

The ideal sensing material would have an electrical response limit asclose as possible to its mechanical limit to maximize the working strainrange, and it has a sensing limit greater than the flexure. Each AMmaterial has its own elastic limit and next is to compare how much ofthe elastic limit the polymer can sense.

An earlier requirement was that the sensor can measure all changes withthe elastic limit of the flexure. The figure above shows that thisparticular polymer cannot sense the full strain limit of the mostpopular AM materials, in some cases only up to roughly 40% in the caseof the SLS nylon series. The electrical sensing range of the silicone isonly 9% of its own mechanical range. The sensing range can be altered bymodifying the doping characteristics as discussed earlier, however anynew formulations would need to be retested mechanically since the twosets of properties are inter-dependent. The measurement range of thepolymer limits the force loading capacity of the sensors since they willrequire a stiffer structure to limit the strain to the upper (orpreferably lower) strain inflection point. It also translates better tosensing stiffer materials like the SLA resins or the FDM plastics,although the SLA Accura 40 is not chemically compatible. This leads tothe conclusion that the FDM series materials are best initial sensorcandidates since they are time-stable, inexpensive, chemicallynon-reactive with the solvent, and the polymer can detect the largestproportion of their elastic limit.

Deflection analysis using a combination of the beam theory and FEAmethods were used to select the geometry dimensions for the first set offunctional sensors of the described embodiments. Samples were builtusing the most prevalent commercial AM technologies and evaluated forperformance on a custom linear servotube testbed for loading profilesand magnitudes up to 10N.

The finite element analysis (FEA) was set up to examine the ABS M30imaterial in linear and bending modes to confirm that the sensor housinggeometry can match the strain limit of the conductive silicone.

The initial design was to maintain a point load at the center of thebeam by keeping the button diameter small. The simulation was carriedout using COSMOS and had a mesh of 66,000 tetrahedral elements, withboth end faces of the tube clamped, a symmetry condition along the X-Yplane, and applied the load on the top of the button normal to thebridge at its midpoint. The loading conditions applied were at the 10Nrange.

The circumference of the button on the dorsal surface (force applicationpoint) was too small, causing the strain to be carried at the centerrather than being evenly distributed throughout the polymer in thebridge. These type of stress concentrations need to be avoided since itcan damage the silicone bridge even at lower loads. By making the buttonwider, it theoretically shortens the length of the sensor ‘L’ because itbecomes stiffer. This is necessary when considering the buckling effectaround the sides of the button seen in the simulation above as well asmaking the strain more even along the bridge and minimizing localizedeffects. For robustness against shear loads it also adds lateralstiffness. The load button radius was equal to r_(o) in this case. FIG.18A illustrates some polymer bridge results for 10N static loads—in thiscase longitudinal strain for the polymer bridge.

Strain Ranges within the bridge based on the FEA are 0.000337 to0.01475. For a 10N load the analysis shows an average strain value ofroughly 0.0078, which is approaching the electrical limit for thepolymer's sensing ability. Although there is a range of strain withinthe polymer bridge, an important consideration is determining what isthe representative value for the entire bridge. The GF equation assumesa consistent and equal strain along the element, and when taking theaverage under the area of the curve, the representative strain is 0.009.This value was used to set the upper load limit of 10N for dynamictesting of the sensor housing.

With the dimensions of the plastic bridge determined, the impact of thepolymer on its mechanical stiffness was examined. The equivalentthickness of the polymer as a layer of ABS M30i plastic was calculatedusing the following equation:

E _(silicone)∈(πr _(i) ²)=E _(M30i)∈(π(r _(n) ² −r _(i) ²))

and overlaid on the existing cross-section as an equivalent thin-walledcylinder of equal elastic modulus. See FIG. 18B for the polymer bridgekey dimensions and composite equivalency. To use as an equivalent Evalue for the silicone, the minimum and maximum values of 0.01 and 0.1GPa were used, as these represent the range for rubber materials.

The equivalent wall thickness was below the minimum suggested buildsettings for using the FDM hardware i.e. even on the smallest featurebuild settings the wall would already be stiffer than any contributionthe silicone would have to resist bending. Thus the simulations coulduse a simplified model of a hollow cylinder because the silicone isnegligible. As a secondary check in reverse, the equivalent wallthickness of the conductive polymer was assumed equal to the minimumbuild wall thickness (which would add another calculation each time asensor according to the described embodiment is built), and then solvedfor what elastic modulus it would have to effect this design constrainton the geometry.

Even if the rubber has an E value identical to the ABS plastic, it wouldstill only be 0.0014142 m thick instead of the minimum wall thickness of0.001762 m; matching the previous assumption. In addition, solving forthe E₂ value to see what E is required to make a significant increase inthe r_(o) it would need to be 1.147 GPa. This is larger than upper Evalue of pure silicone rubber by a factor of 10, which would act morelike medium duty nylon or polypropylene thermoplastic with E between1.5-2 GPa.

The viscous polymer required high pressure when being injected due tothe shear friction along the wall interiors. To account for this, eachinjection port has a Luer lock thread built into the part body tomaintain the seal. FIG. 19 illustrates a force sensor according to thedescribed embodiment with a Leur lock. Once the polymer is fullyencapsulated by the channels, the thread can be cleanly removed bybreaking along a built-in shear line. The copper wire electrodes areimmediately inserted before the polymer begins to gel, and creates arobust electrical contact by pushing aside the neighboring conductiveparticles. Although the bridge can be embedded into any shape ofhousing, the configuration in FIG. 19A is compact enough to build andtest the force sensing principle.

During the build platform preparation phase it was important to adjustthe software's layer slicing parameters. Features with small curves androunds like the polymer bridge can sometimes be interpolated out of thebuild. The original STL file had solid features but the top and bottomsurfaces of the bridge in the bff file were missing because the anglebetween layers was below the default threshold. The final CAD for onesensor constructed according to the described embodiments was able to bebuilt in the SLA Viper with no internal support structure. The buildorientation and build parameters were adjusted to ease post-buildcleaning.

One of the goals for the sensor design was that AM components could bebuilt in nearly any service bureau, and either the end user can insertthe conductive elements themselves or the vendor can easily add it totheir service capabilities. This also necessitates the conductivematerial itself to be readily available and not require any specialhandling procedures outside of normal laboratory and AM safety. Thefilling strategy does not require complex hardware or procedures, andlimits image of filling by using breakaway injection ports. Duringfilling the silicon is able to move through the channels to the sensorsite in its liquid state without deteriorating. The selected material isable to cure within the hollow chamber for the sensor, in conditions ofambient temperature and pressure. Many conductive materials and siliconerubber suspensions were eliminated from selection since they commonlyrequire vulcanizing conditions with high pressures and temperaturesabove the creep values for AM thermoplastics as well as many SLA resins.A commercial injection gun with an 18:1 thrust ratio was modified toreceive and reinforce a polypropylene syringe with Luer fitting.

Samples of a sensor constructed according to the described embodimentswere built from commercial techniques which have plastic materials. TheFDM Parts were built in ABS-M30i (Redeye on Demand, USA). This materialis ISO 10993 certified with biocompatibility suitable for a device withprolonged superficial contact. In addition, the FDM machine selectedutilized a soluble support structure to clear the internal voids of thesensor without damaging it.

The SLA Accura 40 has similar mechanical properties to nylon and is ableto be heat-treated by annealing. The MPJ sample contains multiplematerials and is built with the most rigid and most flexible optionsavailable for the structure and bridge, respectively. A gap wasoriginally left between the flexible button and the rigid housing butduring the build the close edges became fused together with yieldedthree heterogeneous material boundaries rather than two. SLS nylon 12 isalso biocompatible and offers similar properties to some thermoplasticsused in the medial and orthotics industries.

Overall, sensors from each material were able to be fabricated andconduct electricity i.e., fully cure. The SLA samples were the slowestto cure, taking roughly twice as long as the other samples. This is dueto the chemical reaction of the resin during solvent degassing. Thechallenges with the polymer seeping through the layers in the FDM partcould be mitigated by increasing the wall thickness. The MPJ sample hadsome residual expansion since the flexible bridge had unconstrainedfeatures during the injection process. MPJ was also the only family ofsensor samples where failures (bridge ruptures) occurred during filling.The SLS nylon samples has a more coarse finish and thus higher injectionpressures were needed. For future parts with a flexible polymer bridgeit is important to minimize the volume of conductive material whichneeds to pass through during the injection phase.

Immediately following injection while the polymer is still in agel-state, electrodes are inserted on either end of the bridge. Thelocation and alignment of the electrode sites are outside of the polymerbridge to minimize the risk of the graphite particles pulling away fromthe copper leads when the bridge is deflected. Electrically the sensorconstructed according to the described embodiments is closest to astrain gage, or a very low-resistance FSR, and the electrical circuit toacquire the analogue signals from the embodiment is a Quarter Wheatstonebridge type I with a built-in low-pass filter.

The analogue signals from the circuit are taken into the dataacquisition hardware as a floating source differential measurement sincethe variable signal needs to be compared with a respective source whichis not Earth. The other resistors in the Wheatstone were selected tomaximize the voltage change with respect to the R_(s). During sensorrefinement it was observed that the carbon film resistors would overheatover the course of approximately 1 minute, leading to sensitivitydegradation in the measurement. To account for the relatively high200-300 mA current draw, the Wheatstone circuit was constructed usingceramic resistors.

Testing

To examine the electrical response of the polymer inside the bridge ofthe constructed sensor, controlled loads were applied to the center ofthe button, normal to the bridge. Load magnitudes were in series of 2,4, 6, and 10N, which would test up to the theoretical maximum strainwithin the bridge.

The linear dynamometer was built to evaluate the electrical response tomechanical stimuli for testing and calibration of force transducers andload measurement sensors; specifically the piezoresistive response ofthe conductive polymer & sensor specimens. It can apply a static ordynamic mechanical force profile using a Servotube (XSL-230-18, CopleyControls, MA) to deliver a compression force to the bridge unit undertesting, with an off-the shelf precision miniature load cell (LC302,Omega Engineering, Stamford, Conn.) in series for measuring appliedinput force. The response calibration is carried out by correlating theinput force as recorded by the load cell against the output of thespecimen under testing. The servotube is a rod-shaped series ofpermanent magnets which are propelled by current generated in the copperwindings at the center of the electromagnet base mount. The servotubeamplifier has its own built-in PI controller when using the controlvoltage signal. It can operate like this using only the input current tothe servotube, or in closed-loop mode from the load cell measurement.

Each of the loading profiles is generated by a Labview (NationalInstruments, Austin, Tex.) GUI from a desktop computer and uses a BNC2110 DAQ. Dynamic profiles are ramp, square wave, sawtooth, or sinusoid.From the GUI the parameters of the dynamic tests for amplitude, phase,and frequency of each pattern can be set. Both the load cell and sensorhave an analogue low-pass RC filter set for the acquisition rate of 500Hz and a 4^(th) order Butterworth filter in the Labview VI. The rawsensor value (B) is compared to the load cell before and after thedigital filter is applied. A built in 60 second timer automatically runsthe test pattern then records each array of data in a new txt file namedwith the values of its loading parameters.

In addition to force calibration, the system also allows characterizingthe frequency response function for the sensor test sample. The dynamictesting examined sinusoid, square, and sawtooth force profiles foramplitudes between 2 and 10N at frequencies of 2, 4, and 6 Hz using theservotube testbed. Samples were placed near the servotube and coils toconfirm that no interference was coming from the electromagnets.

The Accura 40 samples were unresponsive to all loading profiles andamplitudes up to 10N. The signal was erratic and no perceivable changeswere observed from the loading. This low sensitivity is from thechemical degradation on the inside of the polymer bridge so thatmechanical strain is not necessarily translated to the conductivematerial inside. The only noticeable change in these samples was aftermechanical failure when the polymer bridge was broken and the circuitbecame open.

The MPJ (Multi-Polymer Jetting) samples fully cured and results wereevaluated. For example, FIG. 19B illustrates multi-polymer jetted samplesensor and load cell responses to a 6 Hz sinusoid. The 10N samplesexhibits a dip in the middle of the sine wave responses because of thenon-constant gage factor of the silicone. This shows that the 10N loadstrains the bridge enough to pass the first strain line and almostsaturates the sensor. There is some hysteresis response seen in the 6Nseries between the rise and fall of the load.

Using the MPJ fabrication technique, a single material modality was alsoexamined (VeroWhite) where the entire flexure is made from a singlematerial, rather than the combination of rigid and flexible.

The FDM samples fully cured and results were evaluated. A 30 minutedrift test was conducted for the constructed sensor in the FDM flexureat 2N amplitude and frequency at 0.4 Hz. Signal output was evaluated atthe start and end of the trial.

Both the MPJ and FDM samples showed responses in synchrony with theloading profiles, while varying accuracy and hysteresis. The effect ofthe non-constant gage factor is apparent in the higher-amplitude tests.Two solutions to this effect are to either reinforce the bridge to limitthe amount of strain to within the first linear portion, or to have adata acquisition pattern which examines the past state of themeasurement to determine whether the voltage value is referring to thefirst or second linear strain portions.

Although these results have been obtained for a specific formulation ofconductive silicone suspension, it is not unreasonable to assume thatother solvent-based conductive silicone materials would behavesimilarly. The influence of density of the graphite particles couldpotentially impact the curing time considerably since the solvent iscontained in the volume of silicone suspension, but this would then alsoimpact the mechanical properties of the material performance, as well asthe conductive properties.

A single curve from each was lined up according to timescale, then theMPJ was normalized to match for amplitude with the FDM. The MPJ materialis much softer so returns easier as the servotube was retracting fromapplying the load. This would explain the faster response compared withthe FDM who has a greater spring return but slightly slower responsetime. Additionally, the amplitude is not necessarily the same becausethe different materials are deformed a different amount. The lowresistance of the polymer bridge means that the sensor draws arelatively high amount of current and the resistors in series get hot.The electrical supply and conditions circuit needs to consider heatdissipation because of the high current draw for the polymer. In theWheatstone bridge matching the other resistors with ceramic elements waskey to maintain a power balance after several minutes of continuedoperation.

The response from the drift test was similar to the thermal effect, evenwhen using higher wattage components. When the circuit was powered downand allowed to rest for several minutes it could return with the samefidelity using the same calibration settings. This indicates that thedrift is primarily an electrical or thermal effect rather thanmechanical fatigue.

Some challenges with using the AM materials arise from the speed atwhich they return to their resting positions, and overall visco-elasticbehaviors. Although the MPJ samples had high sensitivity from their lowmaterial stiffness, they were pliable and adhered to the surface of theload cell when the sinusoidal profiles were applied. They also had a lowmaximum force range since their elastic limit is above the conductivesilicone and stiffness is far below the profiles being applied.

Torque Sensor

The injected polymer bridge constructed according to the describedembodiments was applied to a flexure as a customizable torque sensormodality. Commercial torque transducers use bonded metal foil straingages to understand mechanical deformation of the housing. Just liketheir similar counterparts the force sensors, they detect the shearstresses in the torsion bar from an applied torque. The constructedpolymer bridge can also function as a torque sensor if arrayed parallelto the axis of applied torque, the axis of rotation of the sensor. Thedescription herein for a force sensor had design and testing of aspecific force sensor configuration using a variety of AM materials. Thefollowing description addresses the reverse: using a specific materialwhile adjusting the geometric configuration to meet the desiredspecification.

The described torque sensor embodiments use statics principles fromcommercial variations of torque sensors. See, for example, FIG. 20 foran example of such a commercial (Futek) torque sensor. In this case thebonded strain gage conforms to the circumference of the fillets as theystrain, so the design was modified to place four polymer bridges incantilevered shear configuration. To keep it as compatible as possiblethe fasteners and mounting configuration was kept consistent with theFutek.

The electrode configuration and polymer bridge inner & outer diametersare identical to the force sensor design, as well as the injectionmethods using the luer syringe lock. Before getting to a load-bearingversion, fabrication challenges were assessed up front with a ‘Series 1’design built using SLA and FDM and injected with the silicone. At firstthe outer diameter and overall length were kept consistent with thecommercial sensor from Futek since it interfaces readily with the otherdevices which are designed to its geometry. The polymer bridges weremoved to the outside edges of the radius to maximize the shear strainresulting from a torque. Torque Sensor Series 1: Thin polymer bridgeswith the same non-parametric design were used, The SLA version was ableto build but as discussed herein, there were issues with the solventdegrading the inside of the bridges.

Torque sensor series 2: Straight parallel channels kept to respond intension in either direction, with focus on Negative Space rather thanpositive. Fillets were included to remove stress concentration areaswhere the bridges meet the flanges. Examples were built with Small andMedium Loading series (0.2 Nm and 3 Nm respectively).

Varying the angle of the cut between the four bridges adjusts the torquevalue which induces the max polymer strain. The outer diameter wasmaintained even with the commercial equivalent to ease integration withexisting devices. The inner and outer radiuses were kept symmetricalabout the polymer bridge at the minimum.

Dimensions fixed for operations were r_(b) and r_(o) [the former is asclose as we can get to the outside edge for safe build resolution and wecan maximize the strain measured. It is not a good idea to place them atthe interior of the torque sensors since the difference in strainbetween the outside perimeter and the interior where the bridges arewould be strained past the elastic limit. i.e., the exterior would beplastically deformed before the torsional strain had reaching the innermaterial.

Assumptions when modeling the torque sensor FEA were similar as for theforce sensor:

-   -   There is no internal slip between the polymer and the bridge    -   Even average strain is most important along the bridge since the        particle distribution is expected to be homogeneous    -   There is negligible strain at the flanges and no motion at the        electrode—polymer interface

Simulations show that the interface between the fillet and the bridgesare the locations of highest stress and thus highest shear and aresusceptible to failure. They also agree with the calculation assumptionsthat the flanges undergo negligible deformation, especially around thesite of the electrodes. Results for stress distributions are comparablefor both geometries.

The injection fittings normally found on the end of the polymer bridgewere left out of the Series 2 built parts because the wall thicknessrequired for the bend was below the resolution threshold for the FDM andfrom the Series 1 small delaminations were visible which would ruptureif pressurized during injection. Series 2 samples were successfullyfilled via syringe retreating from within the bridges.

After 72 hours measurements were taken every 1 hour for 8 hours toconfirm that the polymer had fully cured and reached its steady stateresting resistance value.

Static and dynamic testing was conducted. The goals of the testing were:(a) confirm the sensors work with the new polymer bridge configuration,(b) examine the sensitivity to the loading ranges advised from the FEAmodels, (c) examine responses to static and dynamic loading profiles.The test setup mechanically grounds one end of the sensor and loads theother end with a mass while measuring deflection angle and load on theend of the arm.

Weights were hung at the end of the load arm on a linear track to guideand ensure that loading was perpendicular to the arm. For dynamicloading, a spring was placed in series with the tensile load cell,elongated to 0.08 m and released to examine the dissipation of energywhile measuring the sensor's decaying oscillations.

To examine the sensor measurement at point of failure, the arm wasstrained sinusoidally with increasing oscillations until mechanicalfailure was pronounced. Measurement continued several cycles afterwards.

Comparing the dynamic tests for the two flexure shapes, the weakerstructure deflects as expected. The Differences in the angle of rotationversus tensile force show that the flexure can be designed of varyingstiffness for anticipated loading ranges while using the same internaldimensions and methodology for the polymer bridge. The dynamicperformance of both sensors was closer to the known values than thestatic testing, possibly from the elastic spring effect in thesinusoidal loading and energy dissipation tests. For the staticcalibration trials, the high standard deviation makes this configurationchallenging for slowly changing loads. However the dynamic performanceis more encouraging, as it was able to detect the peaks of the higherfrequency oscillations when releasing the mass.

Examining the failure mode of the torque sensor, it matches the FEAmodel for stress concentration, and expected magnitude. Looking at theresponse during and after mechanical failure, it is still able tomeasure change in load, albeit at a different signal output range afterthe break. This indicates feasibility for use as a monitoring sensor forthe state of the material since it functioned in torque measurement,point of failure, and strain post-failure.

In some of the manually applied loads it is visible where the polymercrossed from the first linear GF zone into the second (regions A wherethe signal response inverts after crossing the strain threshold).Although it is a consideration for the sensor's operation, the greaterchallenge is that once it had crossed into the second GF zone it did notreturn to the same resting state and had to be left for several minutesto return to its unloaded shape. This is a combined effect of the slowspring return of the FDM material, and some characteristic of thepolymer, likely a persisting strain in the bridge.

Impact Sensing Shear Pin

In the force and torque modalities of the sensor, its responsessignificantly changed after mechanical failure. The Impact sensingmodality uses this property as a conductive switch to indicate when thehousing has broken. For measuring interaction forces to safeguard past aset threshold, the polymer can act like an instrumented shear pinbetween two zones as shown in FIG. 21.

When the polymer bridge is replaced by an unstrained conductive channelwhich passes under shear line on a part, it can operate as a digitalswitch or an analogue sensor to detect the interaction forces betweenthe two moving elements and will open the circuit in the event of amechanical failure. Although designed and tested as a single componentbracket, the conductive shear line could operate as an embedded sensor.As an added benefit for impact sensing for a wearable brace or fail-safefor human-robot interaction device, breaking the pin can activate a callfor assistance from a fall or end the cut power as a hard stop to themachine.

Impact testing was conducted based on the methods in the IZOD notchedImpact Test [reference ASTM D256]. Three groups of five samples werebuilt per material: brackets with and without hollow tubes, with a groupof hollow samples injected with the conductive silicone

L-bracket components were fabricated with and without hollow channelsfor the polymer. The three groups of trials examine the dynamicmechanical effect of removing material from a part cross-section to makeroom for the sensors, as well as the sensing gel's effect on the failuremode of the part. The cross-section of the tube is within 20% of thepolymer bridge sensor, so matches the fabrication capabilities exploredthus far. Five samples of each test group were built for two brittlematerials from SLA and MPJ systems.

The figures below show responses from the sensors during the test.Important to note, these data have no analogue or digital filtering fromthe sensor, showing that at the peak velocity of the swing arm, thevoltage spike is already an indication of impact, followed by a discretestep in voltage when the Wheatstone bridge becomes unbalanced from themissing resistor.

Trials were compared based on velocity after the moment of impact.Although the mass on the end of the moment arm was selected close tostoring kinetic energy to break the samples, the differences in SLA andMPJ were relatively small. The effect of the different cross-sections isshown by the ending velocity after the impact. Compared to the controlgroup's velocity the SLA samples showed no difference by removing thematerial for the cavities, whereas the MPJ groups had some minordifferences by adding the geometry of the channels. The MPJ samples allappeared to absorb energy equally from the swinging mass, indicatingthat addition of the channels and polymer had negligible differencesintra-test.

When examining the mechanical failure mode of the three samples, thegroup with the polymer inside the channels was less explosive for bothmaterials. The two groups without the sensors shattered from the shearline upwards, whereas the sensorized materials broke cleanly into twopieces, in some cases with the polymer still inside the channel, albeittorn. This indicates that the presence of the sensing material doesimpact the energy absorbed at the moment of impact for a damped effect.

Of the available materials, the family of conductive elastomers wassuccessful in building a piezoresistive sensor to measure force in acompact, customizable housing. The process model was successful tocreate sensors injected into AM structures able to sense a variety ofloading profiles and magnitudes.

The Accura 40 can work as a binary state sensor like an on-off switch iffast curing is not required, but is not suitable for an analogue sensor.The MPJ and FDM are both good processes to use, and come with their ownoptions and advantages. The MPJ has wider material selection forflexible structures in the low-force sensing range while the FDM is lessexpensive material which is also biocompatible.

Several AM materials were able to record static and dynamic mechanicalloads under varying conditions, including forces applied directly by ahuman. To calibrate the sensor before use, it would benefit to have somesimilarities to preparing an industrial FSR. Applying a known force fivetimes and adjusting the scale and offset would be routine using an OEMforce sensor applied to the surface button of each sensor constructedaccording the described embodiments that are embedded in the device.

Thus the following summarizes the process steps for creating an AMmedical structure with integrated piezoresistive sensors.

-   -   Scan wearer's surface areas for medical device    -   Select type of force sensor to maximize deflection for sensing        range    -   Integrate CAD for selected sensor geometry into device CAD    -   AM Fabrication    -   Inject Polymer into hollow tubes and insert electrodes    -   Breakaway injection lock fitting

Other embodiments may use conductive material from the graphite siliconegroup. In general, any conductive elastomer which can cure withoutrequiring degassing of a caustic solvent, or at least a smaller amountthen it may be usable for the described embodiments. Adding theconductive elements via injection post-build is in most casesfunctional, but brings unnecessary constraints and errors. Many of thesecomplications could potentially be eliminated by adding the conductivematerial during the build rather than afterwards.

The description below provides exemplary implementations of usingsensors constructed according to the described embodiments in devicesfor upper extremity biomechanics measurement.

The handle design shown in FIG. 22 is a type of hydraulic dynamometerdesigned for a stationary exercise bicycle but can be used as a generalcomputer-interface for retraining. The bike version measures appliedforces to control dynamic motion and steer the rider in a virtualenvironment generated by a computer. The initial prototype isinexpensive compared to alternatives with a compression load cell, andthe built-in compressibility and spring return of the hydraulic chambersprovides a haptic feedback to the rider as they increase isokineticforces. The handle diameter and contours have been selected to providethe greatest ergonomic comfort for grasping while allowing the user tocomfortably maximize their isokinetic strength. It records a measurementfrom dorsal and ventral surfaces but is unable to detect forces fromindividual fingers.

The sensing area of the handle is the surface area of the paddle whichthen contacts the tubing. Tubing under the handle caps is constrainedaccording to the tube minimal bend radius and reorient without kinking.Each channel of the two hydraulic chambers will be embedded alonggrooves in the housing, and thermally bonded together to maintain aclose seal at higher pressures. The handlebars will be calibratedindividually to match the force applied over the tubes to the voltageresulting from the pressure in the hydraulic chambers. FIG. 23illustrates a alternative version of the hydraulic handle depicted inFIG. 22. FIG. 23 utilizes sensors of the described embodiments ratherthan hydraulics.

FIG. 24 illustrates the fabrication and injection stages of the handledepicted in FIG. 23. The four syringe locks (i.e., the Leur locksdescribed herein) are visible in the far left image, and then removedafter the polymer has been injected.

Both devices were tested using the electromagnetic servotube actuatorsetup discussed for testing the force sensor herein. Loading amplitudesranges from 0.5 to 10N of controlled regular compression in sinusoidaland sawtooth waveforms, followed by manually controlling the load.Manual loading was applying tensile forces on the handle surfacesthrough the tension load cell and comparing their results. The sameexperimental setup can be used to load the instrumented devices forbenchtop testing.

The linkage to load the sensing handles was rigid along a linear guide,the underside surfaces of the aluminum force paddles was machined tomatch contact with the sensors. For the hydraulic handlebar it hadsemicircular grooves to match the tubing, for the described embodimentsensors it had two flat surfaces along the length to contact the loadbutton heads.

The first procedure was a static calibration on the hydraulic handlebar.After using the smaller weight to confirm that the calibration values,dynamic testing occurred for sinusoidal patterns for varying amplitudesand frequencies. Each dynamic test lasted 1 minute and the handle wasallowed 1 minute between trials to rest.

The manual input was a load applied by hand on the servotube to pull thetension load cell in series with the hydraulic handle. Although withinthe frequency range of the dynamic testing it was performed to examinethe result of a randomly generated frequency and amplitude from a humanuser.

The manual loading for the dynamic testing of the handlebar with sensorsconstructed according to the described embodiments was applied at theload bar. Both hardware configurations were able to measure static,dynamic, and manually applied loads. The hydraulic handlebar had acomplete characterization and after the 6^(th) iteration is robustenough to be used in a clinical setting. During testing setup there wereseveral higher impact dynamic and impulse loads which could have brokenseals or damaged the hardware but it maintained its performance. Thehandlebar with sensors constructed according to the describedembodiments was able to measure forces and key contacts. The sensor wasable to be embedded into an existing geometry design for a handlebarhousing and acquire and log data. To avoid any interference fromelectrical degradation during testing, the sensor and circuit were givenone minute between trials to rest. Some of the loading peaks and troughswere not picked up by the handlebar with sensors constructed accordingto the described embodiments, more likely due to the mechanical elasticbehavior than the electrical performance since the spring return isslower than electrical dissipation.

The contact between the electrodes and the polymer is a delicateinterface and potential source of noise & signal degradation. Someembodiments refine the insertion of the electrodes to ensure a solidmechanical connection functionally equivalent of soldering a wire to thelead. For human testing, other embodiments include a wider group ofdevices and shapes with which to test specific grasping tasks and handconfigurations using the injected sensor as a modular embedded geometry.

Excelsior is a hand-wrist device for a user post-stroke to measure andassist in hand extension & cognitive repetitive exercises to encourageneuro-plasticity. Details of the Excelsior system may be found in U.S.Patent Application No. 61/566,737, filed Dec. 5, 2011, the contents ofwhich are hereby incorporated by reference herein in their entirety.

The sub-assemblies of the Excelsior system were examined for how thesensing elements could be embedded using less time, fewer components,and less expenditure. The Target Objects and LED Thimbles were bothredesigned using sensors constructed according to the embodimentsdescribed herein.

The force sensing design was modified to fit inside a cylindrical puckdesign. Several of these cylinders were fitted into the cavities of ahollow spherical object design to prehension studies to evaluate thecontact surfaces when performing grasping exercises.

The three pucks were calibrated with static weights before they wereinserted into the spherical object. They functioned when inside theobject, operating independently with three amplifier circuits were ableto independently register contact from the fingertips at varying degreesof exertion. Future work will involve comparing the interaction forcewith a known measurement on each fingertip and specific grasping taskobjectives.

Conductive LED Thimble and Visual Feedback Tools

Fabricating the first version of the conductive thimbles was time andenergy intensive, requiring several stages of preparing the electronics,molding, casting with components suspended inside, and then wrapping thecopper mesh. In the version constructed according to the describedembodiments, hollow channels were designed inside the thimble which actslike a wire. Each thimble is a self-contained circuit connected to abattery. The injection ports for the wires are located under the fingerpad area, and when removed are the two contact points for closing theswitch that activates the LED on the dorsal surface. In one embodimentthe closure occurs when the two contact points come into electricalcontact with a conducting material, such that the two contact points areelectrically connected to one another through the conducting material.In another embodiment the closure occurs because the conductive materialassociated with one of the contact points is situated in a cantileverarrangement such that contact with any material, conductive ornon-conductive, causes the contact points to be in electrical contactwith one another thereby closing the circuit.

This relatively simple implementation is more robust than the originaland fabricated in a single step, with the injection phase following. Asshown in FIG. 25, channels are embedded inside the thimble switch. AnLED is inserted in series with the battery and wires, once the injectionports are broken off. The injection site acts like electrodes of amomentary switch. The conductive thimble can be used as a contact switchor a metallic detector for exercises or games.

In FIG. 26, a two part custom-designed wrist-mounted electronic deviceis shown with embedded channels for implementing conductive paths (i.e.,wiring), on board battery with LED indicator, and magnetic lock. Whenthe channels are not strained the conductive material within thechannels acts as a wire due to minimal time-dependent capacitiveproperties. The hand piece exterior was built as two parts designed froma 3D scan of the mannequin hand. When they encase the hand the circuitcloses. Without straining the conductive polymer it acts like wire. Inthis case for supplying power to an LED to back light an image embossedin the SLA Accura 40 plastic. When the two halves are close enough forthe magnets to secure them, the circuit closes between the two halvesand the indicator light activates. For hand dexterity or balanceexercises which require the wearer to complete a task, visual feedbackcan indicate success. Similarly, the wires can be used to connect anauditory feedback via piezo-buzzer in addition to the LED light.

The graphite suspension has a higher resistivity than pure copper orother metals which make up wires, but can still transmit powerconsistently when injected into a part. Using the same types ofinjection ports and hardware, the tube cross-section can be specified toact similarly to small resistors in series to limit current draw. Thisgives options for resistors being distributed along the wire, and byvarying the doping concentration or cross section achieving differentresistive properties.

The fit of an Ankle-Foot Orthosis (AFO) directly affects its function,including large surfaces and detail features. For example, maintaining ahigh comfort level around the calf band and around the leg are importantso that the fibular head sustains minimal or no pressure. Posterior LeafSpring (PLS) AFOs have a particular trimline configuration which allowsthem to treat drop foot well by reducing plantarflexion during swing.However patients who have severe swelling or edema, unstable ankles, orother ankle-foot deformities cannot use generic posterior leaf orthoticsbecause the mass-produced fit is poor. In addition, patients withmultiple foot ailments need a customized AFO that can be made availableto them quickly for a low cost. In fabricating the RP AFO the aim was tomatch or exceed the effectiveness of a standard AFO in terms ofsupporting and controlling ankle mechanics while providing superiorcomfort and fit by customizing it to the subject's specific anatomy andneeds resulting from impaired gait.

The digital process model expands on traditional orthotic fitting,fabrication, and treatment by preserving the value of experience andquantitative design goals of the orthotist, and minimizing the manuallabor operations and processes which are difficult to record. Automatingfabrication retains geometry selected from key biomechanics parametersby the practitioner while improving speed and availability of a custommedical device. By keeping a digital record of each patient iteliminates the costly need to warehouse physical copies of the legbusts. A long-running record also allows a practitioner to revisitprevious stages of their anatomy and compare changes over time formusculature definition and residual pronation. The physical location ofdata manipulation, fabrication, and data storage are no longernecessarily adjacent to the form capture in the orthotist's clinicbecause all of the AFO modeling stages may be transmitted digitally. Oneof the greatest benefits of the integration of these technologies withthe current orthotics field is that their cost and involvement isscalable to the orthotist. FIG. 27 illustrates a process for creating acustom RP AFO.

Operations were refined for utilizing patient-specific anatomicalsurface data from a 3D scanner, manipulating the surface data to anoptimal form using Computer Aided Design software, and then downloadingthe digital output from the CAD software to an RP machine forfabrication. The new selection of using Nylon 11 powder offered ductilematerial properties similar to the range of polypropylene currently usedby orthotists. In addition the material is time stable and washable, anddoes not leach to the skin for adverse reactions. Gait analysis showedthat an SLS AFO was able to affect the gait of a healthy subject toreduce ankle plantarflexion during gait, which is the normal function tomitigate the dangers of drop foot. Using AM processes for customorthotics components has been investigated for partial or full AFOs invarying degrees, mainly suggesting to use the SLS process with nylon 11powder. At the time of writing, results have not been published for gaitanalysis of an adult who is ambulating with an AFO fabricated entirelyfrom SLS nylon.

A modified scanning methodology was necessary to normalize the surfacesof the ankle-foot complex and minimize the variation in scan data. Anopaque white nylon casting sock can stretch onto the appendage andalmost completely remove all variations is skin tone, whilst decreasesspecular reflection and constraining the flesh. Potential problems fromhair are thus also eliminated without having to shave the appendage.When stretched, the stocking adds a thickness of 0.25 mm to the skinsurface.

The ankle-foot complex was in subtalar neutral referring to the relativeorientation of the shank and foot. The posture of the lower extremitieswere slightly supine (leaning forward and supported) to allow thescanner to observe the ventral surface of the foot and posterior side ofthe leg in the same field of view. Scan anomalies and poor-fittingcontours are removed by local curvature maximum comparison and Gaussianhole-filling algorithms for each individual point cloud. The clean pointclouds are then merged into a single point cloud and a surface mesh isfitted. During a scan, data is captured which is relevant to the patientanatomy, as well as extraneous data from the environment and theorthotist's hands which must be removed. By creating a large contrast incolor between the patient's anatomy and all other points the unwanteddata may be removed according to range of hue & saturation for thevoxel. The white balance from the patient's sock-covered appendage has ahigh contrast with the orthotist because of their blue gloves. Anysurfaces occluded by the blue gloves cannot be registered from a scan,but may be added from a separate mesh captured when the practitioner'shands have moved to a different location on the patient's ankle.

A significant amount of subjective surface manipulation is required todevelop the AFO shape model in both physical and digital processes. Themodifications to the AFO digital scan still use the orthotist'sinstructions for location and offset distance of each region. 3Dmanipulation software like Rapidform has the capacity to perform surfaceoverlay deviation analysis to compare the surface of the leg scan, withthe cleaned, modified, and parameterized AFO digital model.

FIG. 28A illustrates the flow diagram of digital processes for pointcloud refinement (AFO Digital Model Refinement Stages).

During each cleaning procedure it was important to evaluate byconducting inspection & analysis the continuity of the AFOs curvatureplot to detect local irregularities. The surface mesh should ideally bea continuous smooth surface. Once the surface is well-behaved a patch offour sides NURBS model surfaces is fitted to the mesh in preparation forfunctioning as a CAD equation-driven feature reference. Once in CAD thesurface may be referenced to offset features, thicken surfaces, createcavities, or extrude features.

A non-sensorized AFO using Polyamide Nylon 11 “Duraform PA” wasfabricated to examine the process challenges using an SLS process tofabricate the AFO as well as its dynamic effects on a healthy subject'sgait. Once the thermoplastic is fully sintered there is no inhalationhazard and can be autoclaved or washed. This material is rated as aUnited States Pharmacopeia (USP) Class VI biocompatible material, whichposes no danger to superficial contact and can even be implanted withinthe body for up to 30 days. The mechanical properties of sintered nylonare within the range of extruded polypropylene materials currentlyemployed by orthotists. Additionally, the thermoplastic materials usedby both FDM and SLS may be selectively heated and their feature surfacesadjusted before the material re-cools and solidifies.

The trimline configuration and material thickness (3 mm) were set tomatch the semi-flexible polypropylene AFO. To optimize the mechanicalmaterial performance of the AFO and Z-axis, the build orientation wasset to maximize the tensile yield point by aligning the horizontal datumalong the Achilles. The AFO used was built in a P730 SLS system (EOS,Novi, Mich., USA). The height of this AFO was designed roughly 15%shorter than what would normally be prescribed in order to accommodatethe SLS build platform available. Of the regions contacting the leg, thefit was comfortable.

Orthotists keep a detailed dialogue with each wearer and will usuallyhave them ambulate for 20 minutes inside or near the clinic afterdonning a new AFO. Although a formal gait analysis is the mostquantifiable assessment of benefit, even just ten minutes of casual gaitreveals regions with problematic/uncomfortable fit. Locations andseverity of blisters, pinching, swelling, or redness are all commonindicators of the fit comfort.

The lateral trimlines set the level of rigidity in the AFO and areprescribed to the patient by an initial gait analysis. The slope andtaper of the trimlines is assessed primarily qualitatively for thepatient's needs. Two custom polypropylene AFOs were fabricated using theconventional process. AFO A is an off-the-shelf polypropylene posteriorleaf spring orthosis and was sized from nearest available fit. AFOs B &C were fabricated based on trimline contours to give greater (flexible)freedom in dorsi & plantarflexion angle and less (semi-flexible) freedomfor range of motion. The role of an AFO in gait is to allow a specifiedrange of motion to increase gait symmetry and cadence range with maximumcomfort and minimal increase in the wearer's energy expenditure. Thetrials in self-selected cadence, ankle range of motion and ankle energywas compared for gait between RP & traditional AFOs, both built customfor the wearer.

To characterize the gait pattern of the subject reflective markersplaced with on the following specific anatomical landmarks of thesubject's pelvis, and knee, ankle and foot of each leg. Additionalmarkers were also rigidly attached to wands and placed over themid-femur and mid-shank. The subject was instructed to ambulate along a20 foot walkway at their self-selected comfortable speed for all of thewalking trials. An 8-camera motion capture system recorded thethree-dimensional trajectories of the reflective markers during thewalking trials. Two force platforms embedded in the walkway surfacerecorded the three-dimensional ground reaction forces and moments duringfoot contacts onto the platforms.

The subject was a right-foot dominant healthy adult with no previousambulatory or cognitive deficits and wore the AFOs on their right side.Four different conditions were tested during the gait evaluations: 1)with sneakers and no AFO (No AFO); 2) with the standard polypropyleneposterior-leaf spring AFO (PP PLS); 3) with the flexible custom AFO (PPFlex), 4) and with the custom RP AFO (SLS RP). Each of the differentAFOs was fitted to the right leg of the subject during the level walkingtrials. Five walking trials with foot contacts of each foot onto theforce platforms were collected for each AFO condition.

Both ankle dorsi and plantarflexion angles are comparable with closestandard deviations. Data from each brace condition was compared withcharacteristics of the right leg with No AFO. Range of motion for theSLS AFO exhibits comparable patterns and magnitudes to normal gait, aswell as symmetry between the left and right ankle. Dorsiflexion angledoes not appear to be reduced as significantly as the other AFOconditions, although plantarflexion has decreased. Cadence wasconsistent between trials and comparable to gait with No AFO, indicatingthat this brace condition did not significantly inhibit ambulation.

Moments about the right ankle for No AFO matched magnitudes and temporalpatterns with normal gait. For the PP PLS and PP Flex AFOs the shape ofthe moment curve matched the No AFO condition for peak location, butmagnitudes have decreased by 24% and 18% respectively. Power generatedat the ankle was significantly reduced for both PP PLS (84%) and PP Flex(57%), but exhibits the same patterns as healthy gait. This is asexpected from the material resisting motion, which can also be a benefitto patients requiring increased stability during powered plantarflexion.Both AFOs show a higher standard deviation around peak power just priorto heel strike, again indicating an inconsistency in gait patterns. Thismay be due to greater compliance of the polypropylene material fromwhich the standard AFO is made or a poorer fit of the AFO around thefoot and ankle of the subject compared to the custom PP Flex AFO.

When comparing the two traditional (PP) AFOs they perform similarly interms of controlling ankle kinematics and kinetics during the gaitcycle, with some small deviations according to material and trim line.The SLS AFO has some small effects on gait, primarily in the peak ankleangles being reduced slightly but has a lesser impact than either of thePP versions or the off-the-shelf condition. The greatest likelycontributing factor is because it is shorter than the other customdesigns it is unable to hold and stiffen the ankle with its smallermoment arm. This is also seen in the graph for ankle power in the rightside since it is not altering the power release of the ankle duringtoe-off as compared with the no AFO condition.

The process to build a SLS AFO was successful in comfort and had somebiomechanics impacts, even at a reduced design height. If worn over alonger period of time, this smaller version would become uncomfortablebecause of the upper edge digging into the leg halfway along theAchilles. A pair of new AFOs was designed: a solid version with nochannels and one instrumented with channels to receive the conductivepolymer according to the described embodiments. The base design would bemodified to match the actual polypropylene AFO in posterior height tohave a relevant impact on gait, and have options for versions with andwithout the hollow tubes for the sensing silicone.

Even with the custom SLS version, there is need of a way to know at whatstage of an AFO's lifespan it is currently in. By using the SLS processas the method of fabrication, there is relatively little overheadworkload or complexity added by embedding sensing elements. The AFO mayhave begun to deform plastically without having significant visualimpact or significant feel to the wearer. Serious injury such as fallsor tripping can occur if it unexpectedly fails during use. Embeddedsensors may offer clinicians and patients data on the state of thefatigued AFO over the course of its lifespan.

FIG. 28B shows the overview of the sensor and AFO functions interactingwith the wearer. At the initial state the AFO performs well to assistand stabilize the gait of the wearer. The range of electrical resistance(red lines) is initially a low magnitude range of strain (blue arrows).Over time the AFO mechanically fatigues, allowing the range of strain toincrease. When the sensor resistance values pass a threshold to indicateplastic deformation of the AFO, it signals that it has reached the endof its safe operational life span and prompts the wearer to have itinspected or replaced. This also allows for iterating design andgeometry changes as necessary based on patient feedback, biomechanicalanalysis of the device and its wearer, and analysis of the measurementstaken over time by the embedded sensing elements. These iterations couldmean modifying the thickness of the material, the trim lines indicatingthe edges of the material, locations of the embedded components, densityof the material generated during the fabrication process, etc.

When compared to other available sensor types, the piezoresistive-basedsensors constructed according to the described embodiments was theeasiest to integrate because it could accommodate deformations which aretoo large for strain gages, while readily fitting into hollow cavitieswithin the AFO material. An alternative used in SDM techniques isFiber-Bragg Grating-sensors, which are an excellent strain-sensingchoice free from electro-magnetic interference. The obstacle to usingsuch technology in this application is inserting the glass tube throughchannels within the AFO. This entails conflicting requirements ofmaintaining a clearance gap between the glass and material body, yet tohave no clearance gap to keep the sensitivity high. For redesign usingthe sensors of the described embodiments, material is removed in CAD tocreate the channel voids in order to make space available to fill withthe conductive gel. The two versions of the SLS AFO were necessary toexamine the biomechanics effect of removing this material as a tradeofffor sensing.

Instrumenting the AFO builds on the process of generating an RP AFO vianon-contact 3D scanning and stereolithography. FIG. 28C illustrates aprocess diagram for creation, instrumentation, application and loggingof a custom sensorized AFO.

Determining the appropriate insertion sites is needed for earlynotification of AFO fatigue. Breakdown usually occurs around the base ofthe Achilles where the stress concentration is highest. A finite elementmodel of the AFO geometry was examined in bending (using the torque andangle parameters from normative ambulatory data) to identify straindistribution. Simulations were examined for the AFO response to momentsapplied through the ankle joint at an axis parallel to the coronalplane. Using tetrahedral elements, a mesh of 49,717 nodes was generatedin Cosmos (Dassault Systemes, Waltham, Mass., USA) for the FEA.

Simulation Limitations:

-   -   The dominant motion of the ankle is in the sagittal plane so the        analysis was performed flexing in this plane.    -   Although the axis of rotation in the physical ankle alternates        between dorsi and plantarflexion during gait, the axis of        applied moment in the model remained normal to the sagittal        plane.

Using the isotropic material properties of Nylon 11, loading wassimulated to examine the way strain propagates through the posteriorsurfaces according to the magnitude of applied moment. Transferring thesensor geometry to the measurement sites in the AFO is selectedaccording to the regions exhibiting mechanical strain within the sensingrange. As the strain region expands and increases in magnitude, the edgeof this embodiment should be close enough to register the strain withoutbeing stretched past its sensing saturation point of 0.0863 strains. Thetube cross section of 2 mm was selected to ease injection with a largerdiameter while retaining 0.5 mm wall thickness on either side of the AFOfor build resolution.

Additional benefits of using CAD to design the AFOs is easilyintegrating mounting hardware and fixtures. Four mounting holes arebuilt into the top to house electronics or attach an OEM cuff. This ischosen at the upper extremity of the shank support to avoid impactingthe AFO stress distribution at the base of the Achilles where the sensoraccuracy is most needed. Two sensing channel designs were included alongthe posterior reaching into the edge of the strain zone. The medial andlateral tabs were removed from the design to reduce build time andmaterials cost. The analysis conditions were also re-run to iterate theending location of the channels. FIG. 29 illustrates the posterior viewof AFO CAD with cavities (right edge) and calf tabs removed. Channellocations were updated using FEA from this new design.

After removal from the AM fabrication chamber, special attention wastaken to clear out nylon powder material from the cavities and interiorfeatures for the sensors. Sensing and data transmission components areinserted into hollow cavities or attached via fasteners. The AFO surfacewas bead-blasted to remove clinging powder. FIG. 30 provides acomparison of three AFOs. The traditional polypropylene AFO, thenon-instrumented SLS AFO, and the SLS AFO instrumented according to thedescribed embodiments. The table below provides further comparison forthe three AFOs.

SLS AFO SLS AFO Physical Property PP AFO No Channels With Channels Mass(g) 200 235 230 Material Thickness (mm) 2.3-3.63 4.1 4.1 Height atposterior (cm) 35.87 34.59 34.62 Posterior Leaf Width Min (cm) 4.16 5.185.18

FIG. 31 shows the feature detail for the channel injection sites on theAFO instrumented according to the described embodiments. The resolutionof the hollow channels shows the interior along the AFO. The sensingchannel acts in this case like the tensioned side of a cantilevered beamin bending.

Gait evaluation used a combination of IR motion capture and externalload cells in the gait walkway. Each of the cameras of the Vicon systememit strobed IR light, which when reflected gives a grayscale view ofeach marker in 3D space. The co-ordinate of each marker is thencalculated within the camera from triangulation of the markers andautomatically tracks the markers to establish 3D trajectories usinginverse kinematics. The procedure to record kinetic & kinematiccharacteristics consists of attachment of retro-reflective markers onkey anatomical joint positions of the pelvis and lower extremities, andambulating along the walkway registering one heel strike per foot perforce platform. Reflective markers were placed on the pelvis and lowerextremities using the same process as the first version of the SLS AFO.A static measurement was taken between each brace condition to accountfor any shifting of the markers. The AFOs were worn on the right leg asthe ‘affected side’ with the left leg as the ‘unaffected side’.

The gait kinetics and kinematics were recorded between four braceconditions:

-   -   No AFO (sneaker gait)    -   Traditional AFO    -   SLS AFO with no Sensor Channels    -   SLS AFO with Sensor Channels.

The traditional AFO was fabricated from polypropylene thermoplastic byan orthotist, aimed to provide flexible support and some resistance indorsi and platarflexion. The two SLS versions were designed from a 3Dscan according to similar trimlines of a semi-flexible PLS AFO. Thegoals of the baseline biomechanics testing were as follows:

-   -   Establish baseline biomechanics effect of each AFO on gait when        they are brand new    -   Examine the magnitude of difference between the two SLS versions        as attributed to the channels.

The gait pattern of the test subject has some graphs like ankle angleand ankle moment outside of the baseline measurements compared to thenormal population data but is still considered to have a healthy gait.Temporal parameters for the trials are summarized in the table below.

Opposite Cadence Walking Opposite Foot Foot Stride (steps/ Speed FootOff Contact Off Length min) (m/s) (%) (%) (%) (%) No L Avg 101.57 1.1912.26 48.52 62.90 1.41 AFO StDev 1.71 0.03 0.99 0.31 0.30 0.01 R Avg99.34 1.19 14.21 50.62 64.82 1.43 StDev 1.77 0.04 0.19 0.74 0.34 0.02Tradi- L Avg 94.76 1.08 13.42 49.21 63.82 1.36 tional StDev 1.76 0.050.62 0.29 0.40 0.03 R Avg 93.92 1.09 14.47 50.34 64.27 1.39 StDev 2.430.04 0.17 0.60 0.70 0.02 Printed L Avg 93.40 1.03 14.14 49.93 64.33 1.32No StDev 1.31 0.03 1.29 1.01 0.56 0.03 Channels R Avg 94.01 1.07 14.4950.39 63.97 1.36 StDev 1.18 0.01 0.43 0.63 0.42 0.02 Printed L Avg 94.991.06 12.93 49.87 63.98 1.34 with StDev 0.56 0.02 0.64 0.44 0.40 0.03Channels R Avg 94.87 1.10 14.09 50.07 62.98 1.39 StDev 1.21 0.02 0.630.28 0.47 0.02

The ankle angles are in greater dorsiflexion during heel strike andstance and less plantarflexion than the population, with peak toe-off atthe average or higher end of normative data. The subject approaches heelstrike with the ankle angles in more dorsiflexion than the normativepopulation and has natural toe off roughly 5% later in the gait cycle.Possibly the subject takes larger steps than the normative population asshown by the heel strike and toe off angles of the sneaker (no AFO)gait. The natural difference between the left and right sides showshigher dorsi and plantarflexion angles for the right than the left side.

All brace conditions significantly decrease the plantarflexion range ofmotion in the right side, without significantly modifying the temporalpattern of the gait events. This is consistent with the effect of AFOsto prevent dropfoot. The peak angle during dorsiflexion stance isrelatively unchanged between all AFO conditions but the angle duringheel strike is decreased by 50% or more. Likewise, the peak knee anglesfor stance and swing are both markedly smaller during any of the braceconditions, likely because the stride length had decreased. All braceshave similar effect on knee angle, with peak flexion taking placeslightly later in the gait cycle (similarly with the subject's late toeoff normal gait)

It is also possible that the unaffected (left) side has somecompensatory strategies since the peak ankle plantarflexion angleincreases when the contralateral side is wearing a brace, and the peakknee flexion angle also increases by 3-5°, but is not conclusive sincethis value is within the standard deviation of the trials collected.

Ankle moment for the unaffected side is relatively unchanged but stillremains on the higher end of the normative population, with peak momentslightly higher than the standard deviation. The subject's right sidehas a natural increase in power at 20% through gait which is notentirely unusual [reference Neptune gait paper here] but is not seen inthe normative population data. This bump becomes mitigated when wearingany of the AFOs, as well as peak moment also decreasing; possiblybecause of a slower walking velocity from wearing the braces. The tablebelow illustrates peak gait values for right ‘affected’ side wearingAFOs.

Ankle Ankle Power Knee Ankle Angle Moment During Stance Knee AngleMoment Brace (deg) (Nm/kg) (W/kg) (deg) (Nm/kg) Condition Dorsi PlantPeak Peak Trough Flex Exten Peak Sneaker 17.54 12.91 1.94 3.77 1.4771.18 10.17 0.50 Traditional 18.99 3.04 1.85 2.55 1.28 61.78 3.76 0.24AFO SLS AFO-No 18.11 0.14 1.72 2.12 1.05 66.38 7.00 0.32 sensor SLS AFO-17.14 1.23 1.77 2.51 1.28 60.38 4.51 0.22 with sensor

The two SLS AFOs have similar impact on ankle angles within 2° of eachother to decrease peak dorsi and plantarflexion, and even have similarplantarflexion angle to the traditional version. The polypropylene ismore flexible than the glass-filled Duraform EX, and its impact showsthis difference in decreased heel strike and toe off angles. Peak kneeangle in the right side for extension and flexion are similar, with the‘SLS sensor’ version closer in effect to the traditional than the ‘SLSnon sensor’. The hollow cavities of the former will make the AFOslightly less stiff than if it were hollow. i.e. more closelyapproximate the more flexible traditional version. It is worth notingthat although more flexible than the ‘SLS non sensor’ it is stillstiffer than the traditional version, as indicated by peakplantarflexion angles during toe off between the three. In thecontralateral side, knee angle and moment compensatory effects seem tobe almost identical between the three brace conditions.

The impact on gait from the SLS AFOs was within the ranges of thetraditional AFO for both legs. The version with hollow cavities toreceive the polymer did behave with slightly more flexibly during peakangles but was a medium effect between the traditional andnon-sensorized designs.

The ambulatory impact and mechanical state of the AFO can be assessed atthe start and end of its anticipated lifespan. The parameters from thefirst gait analysis will be inputted to a motor controller in the AFOtestbed to apply torque and wear down the AFO to simulate wear of up to24 months of use. Adjustable clamps and platforms allow the axis ofrotation of the testbed to be coincident with the AFO (about the base ofthe tibia bone). A hinged surrogate leg design is used to apply a momentprofile the same way as the wearer's leg as in some similar otherdesigns.

Strain gages bonded to the posterior surfaces coincident with thepolymer sensor sites will all take periodic data to examine the strainstate of the material, as well as compare the performance of the twosensor types. A rotary encoder inside the motor gearbox can measure AFOangle of dorsi/plantar flexion, and a load cell at the AFO/surrogateinterface is able to measure interaction forces as it resists theloading.

Considering that the SLS AFO with hollow cavities performed in betweenthe SLS solid and traditional versions it is feasible for it to matchthe mechanical properties of certain variants of traditional AFOs evenwith the material removed for the sensing material. With furthercharacterization of its mechanical effects using the AFO testbed, it maybe possible to approximate the impact of the traditional version moreclosely while also having the added benefit of sensors embedded insideit. The surface finish of the Nylon 12 is significantly rougher becauseof the glass beads, which poses challenges to injecting material over achannel running almost the entire length of the Achilles.

As a process for refining the advantages of an AFO build using AMtechniques, combining the custom processes with modular design andadjustable fittings is a strategy considered for future work. The onlycustom-built regions are around the ankle which stabilizes the bonyregions, while using off-the shelf solutions for the cuff which wrapsaround soft tissue. Finally, to offer a modular mechanical stiffness andspring return, an interchangeable load beam at the posterior connectsthe two regions.

Finally, to offer a modular mechanical stiffness and spring return, aninterchangeable load beam at the posterior connects the two regions.This follows the design intent of “only custom build the bare minimumnumber of components.

The described embodiments may also utilize AM hardware that fabricatesthe electronics inside the parts as they are being built. For example,instrumentation that traditionally would be located remotely from thesensors could, at least in part, be embedded along with the sensoritself. Components such as amplifiers, filters, comparitors, buffers,and other such elements associated with data acquisition and measurementcould be embedded.

Other embodiments may include the following improvements:

Particle Optimization Studies

The process to create the conductive elastomer merits refinement forparticles with more uniform shape and size. This would improverepeatability and homogeneity of the volumetric resistivity. Thetradeoff for higher density of conductive particles is a more brittlemechanical properties and lower elastic limit. Examining other types offormulations for elastic silicone could increase the elastic limit for alarger range of elastic deformation. Other types of silicone which havea smaller amount of solvent (reaction inhibitor) escaping could decreasethe curing time.

Examine Alternative Materials which do not Contain Nickel

The current choice of using a conductive element which contains Nickelmay not be used in some embodiments. Nickel has a non-linear gage factorwhich can be very challenging to model when back calculating the forcefrom the transducer's response. In addition, there are questionablelong-term health risks for using Nickel which should be avoided. Fineground graphite powder in a silicone RTV suspension may be used in someembodiments. Additionally, graphite by itself is non-toxic and severalconductive material combination have been presented made frombio-friendly and even household ingredients.

Increase the Number and Density of Sensing Channels

In some embodiments, linear and circular arrays of the same channelcould increase the resolution of force sensing and improve detection ofnon-normal (tangential) loading on the sensor. Layers of matrices mayincrease the maximum force sensing saturation point by layeringlow-force (high strain) sensors on top of high-force (low strain)sensors to detect a similar ranges of forces but in a greater number ofaxes.

Sensing Layer for Protective Padding in Sports & Hazardous Work for theWrist, Ankle, Neck, or Head.

In some embodiments, a thin AM layer of wearer-specific force sensorsbetween the human and their exterior creates a sensing ‘carapace’ todetect impact forces as an early warning system for injury. A helmet maybe instrumented with sensors to detect impact forces on particularlocations of the skull for early warning of concussion or head trauma.

Pre Instrumented Components for Small Mobile Robots

Small flying/walking robots (as well as robotic devices) can bemission-customized for sensors and self-diagnostics in their endeffectors, limbs, and internal mechanisms. A standard core unit withmicrocontroller, power, and communications can connect to theseappendages which are mechanisms built on demand with sensors andelectronics together. FIG. 32 illustrates a robot wing with an embeddedstrain sensing and a robotic leg with embedded sensors to detect impactfrom ground reactive forces. The described embedded sensors in roboticcomponents could be extended to full size, non-robotic components, e.g.,full sized airfoils on aircraft.

To monitor the state of limbs and manipulators in small robots, embeddedsensors according to the described embodiments can help for detectingfoot contact during locomotion when along the underside of the foot, orbe contained within the body to detect damage in the limb after impacts,collisions or falls. A central controller in the body can be connectedto the peripheral elements as if the mechanical structure is also asensing structure, taking into account the range of motion of the limbs.

Thin airfoils can benefit from embedded sensors to monitor windturbulence and health of the structures. The scale is meant for scalemodel aircraft or remote-controlled hobby size as opposed to acommercial airliner. The shape of airfoils is already a precisely-builtfreeform surface, by using additive methods to prototype the structure;sensors according to the described embodiments can give feedback toengineers for test models in the wind tunnels to validate the simulationmodels. The image shows the location of the sensors at the wing tips,this is an example of examining where the greatest strain would occur.For oscillations resulting from turbulent flow this could also be anindicator. For detecting undesired strain at other key locations likewhere the wing attaches to the aircraft body, the sensors of thedescribed embodiments can warn in case plastic deformation is occurringin case of high loads.

Vibration Sensor Using a Mass on the End of a Cantilevered PolymerBridge

By affixing a mass on one end of a cantilevered beam, acceleration ofthe sensor will cause the polymer bridge to bend. The frequency andmagnitude pattern of the strain induced from this bending can be backcalculated for acceleration.

Patient-Specific Cuff to Connect with a Robotic Exoskeleton

The role of robotic exoskeletons in physical therapy is currently beingexplored. Their modes of operation in this field are still beingexamined, but regardless of the outcomes, a comfortable and securepatient-robot physical interface will be of great importance. Thetechnology of the described embodiments can be used to generateinstrumented patient-specific cuffs based off of a 3D scan of the areaof interest. The areas below the knee and behind the thigh are popularchoices for lower-limb interfaces since they have close contact to thebone and can generate the largest torques (respectively). The ankle-footcomplex is used to mechanically ground the exoskeleton and can alsobenefit from embedded sensing to monitor the tissue compression andstrain of the uprights connecting the robot.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of the equivalency ofthe claims are therefore intended to be embraced therein.

What is claimed is:
 1. A sensor, comprising: a structure, including oneor more voids distributed therein; a material deposited within the oneor more voids, wherein the material is characterized by one or moreelectrical properties; a first contact electrically coupled to a firstlocation on the material; and, a second contact electrically coupled toa second location on the material.
 2. The sensor of claim 1, wherein thestructure includes a plurality of consecutive layers, each of which is across-sectional profile of the structure.
 3. The sensor of claim 2,wherein the plurality of consecutive layers was produced using anadditive manufacturing technique.
 4. The sensor of claim 1, wherein thestructure is based on a sensor design.
 5. The sensor of claim 4, whereinthe sensor design describes a torque sensor.
 6. The sensor of claim 4,wherein the sensor design describes a force sensor.
 7. The sensor ofclaim 4, wherein the sensor design describes an impact sensor.
 8. Thesensor of claim 4, wherein the sensor design describes a bend sensor. 9.The sensor of claim 4, wherein the sensor design describes a vibrationsensor.
 10. The sensor of claim 1, wherein the first location on thematerial is a first end of the material and the second location on thematerial is a second end of the material.
 11. The sensor of claim 1,wherein the one or more electrical properties includes piezoresistiveproperties.
 12. The sensor of claim 1, wherein the material is depositedwithin the one or more voids by injecting the material through anopening in the structure.
 13. The sensor of claim 12, further includingan adapter connecting the opening to an injector, wherein the adapterincludes threads to couple to the injector.
 14. The sensor of claim 13,wherein the adapter is removably coupled to the opening in thestructure, such that the adapter can be detached from the opening afterthe material is deposited in the void.
 15. The sensor of claim 1,wherein the material includes graphite particles in a silicone RTVsuspension.
 16. An orthotic device, comprising: a structure forproviding support to a portion of human anatomy, the structure includingone or more voids distributed therein; a material deposited within theone or more voids, wherein the material is characterized by apiezoresistive property; a first contact electrically coupled to a firstlocation on the material; and, a second contact electrically coupled toa second location on the material.
 17. An ankle-foot orthosis,comprising: a structure for providing support for one or more of a foot,ankle and lower leg, the structure including one or more voidsdistributed therein; a material deposited within the one or more voids,wherein the material is characterized by a piezoresistive property; afirst contact electrically coupled to a first location on the material;and, a second contact electrically coupled to a second location on thematerial.
 18. An upper extremity measuring device, comprising: astructure having a first surface and a second surface, the structureincluding at least one void distributed within the structure beneath thefirst surface and at least one void distributed in the structure beneaththe second surface; a material deposited within the voids, wherein thematerial is characterized by a piezoresistive property; and, for each ofthe voids within the structure: (a) a first contact electrically coupledto a first location on the material; and, (b) a second contactelectrically coupled to a second location on the material.
 19. A devicefor sensing contact with an object, comprising: a structure having anexterior surface, the structure including at a first void and a secondvoid extending into the exterior surface, wherein the structure includesa plurality of consecutive layers, each of which is a cross-sectionalprofile of the structure; a material deposited into the voids, whereinthe material is characterized by a piezoresistive property and whereinthe material deposited into the first void is not in contact with thematerial deposited into the second void; an electrical circuitelectrically coupled to the material deposited into the first void andto the material deposited into the second void; wherein the exteriorsurface contacting the object causes the electrical circuit to form aclosed electrical circuit.
 20. The device of claim 19, wherein aconductive object causes the electrical circuit to form a closedelectrical circuit when the conductive object is electrically coupled tothe material in the first void and to the material in the second void.21. The device of claim 19, wherein the object causes the electricalcircuit to form a closed electrical circuit when the object manipulatesa cantilevered portion of the material in the first void to beelectrically coupled to the material in the second void.
 22. The deviceof claim 19, wherein the plurality of consecutive layers was producedusing an additive manufacturing technique.
 23. A device for supportingat least a portion of an electrical circuit, comprising: a structureincluding one or more voids distributed therein, wherein the structureincludes a plurality of consecutive layers, each of which is across-sectional profile of the structure; a material deposited into theat least one void, wherein the material is characterized by apiezoresistive property; wherein the material is electrically coupled tothe electrical circuit, such that the material forms at least a portionof a conductor in the electrical circuit.
 24. The device of claim 19,wherein the plurality of consecutive layers was produced using anadditive manufacturing technique.